Delineation of Nature. General Review of Natural Phenomena (part 1)

WHEN the human mind first attempts to subject to its control the world of physical phenomena, and strives by meditative contemplation to penetrate the rich luxuriance of living nature, and the mingled web of free and restricted natural forces, man feels himself raised to a height from whence, as he embraces the vast horizon, individual things blend together in varied groups, and appear as if shrouded in a vapory vail. These figurative expressions are used in order to illustrate the point of view from whence we would consider the universe both in its celestial and terrestrial sphere. I am not insensible of the boldness of such an undertaking. Among all the forms of exposition to which these pages are devoted, there is none more difficult than the general delineation of nature, which we purpose sketching, since we must not allow ourselves to be overpowered by a sense of the stupendous richness and variety of the forms presented to us, but must dwell only on the consideration of masses either possessing actual magnitude, or borrowing its semblance from the associations awakened within the subjective sphere of ideas. It is by a separation and classification of phenomena by an intuitive insight into the play of obscure forces, and by animated expressions, in which the perceptible spectacle is reflected with vivid truthfulness, that we may hope to comprehend and describe the 'universal all' [Greek words] in a manner worthy of the dignity of the word 'Cosmos' in its signification of 'universe, order of the world', and 'adornment' of this universal order. May the immeasurable diversity of phenomena which crowd into the picture of nature in no way detract from that harmonious impression of rest and unity which is the ultimate object of every literary or purely artistical composition.

Beginning with the depths of space and the regions of remotest nebulae, we will gradually descend through the starry zone to which our solar system belongs, to our own terrestrial spheroid, circled by air and ocean, there to direct our attention p 80 to its form, temperature, and magnetic tension, and to consider the fullness of organic life unfolding itself upon its surface beneath the vivifying influence of light. In this manner a picture of the world may, with a few strokes, be made to include the realms of infinity no less than the minute microscopic animal and vegetable organisms which exist in standing waters and on the weather-beaten surface of our rocks. All that can be perceived by the senses, and all that has been accumulated up to the present day by an attentive and variously directed study of nature, constitute the materials from which this representation is to be drawn, whose character is an evidence of its fidelity and truth. But the descriptive picture of nature which we purpose drawing must not enter too fully into detail, since a minute enumeration of all vital forms, natural objects, and processes is not requisite to the completeness of the undertaking. The delineator of nature must resist the tendency toward endless division, in order to avoid the dangers presented by the very abundance of our empirical knowledge. A considerable portion of the qualitative properties of matter — or, to speak more in accordance with the language of natural philosophy, of the qualitative expression of forces — is doubtlessly still unknown to us, and the attempt perfectly to represent unity in diversity must therefore necessarily prove unsuccessful. Thus, besides the pleasure derived and tinged with a shade of sadness, an unsatisfied longing for something beyond the present — a striving toward regions yet unknown and unopened. Such a sense of longing binds still faster the links which, in accordance with the supreme laws of our being, connect the material with the ideal world, and animates the mysterious relation existing between that which the mind receives from without, and that which it reflects from its own depths to the external world. If, then, nature (understanding by the term all natural objects and phenomena) be illimitable in extent and contents, it likewise presents itself to the human intellect as a problem which can not be grasped, and whose solution is impossible, since it requires a knowledge of the combined action of all natural forces. Such an acknowledgement is due where the actual state and prospective development of phenomena constitute the sole objects of direct investigation, which does not venture to depart from the strict rules of induction. But, although the incessant effort to embrace nature in its universality may remain unsatisfied, the history of the contemplation of the universe (which p 81 will be considered in another part of this work) will teach us how, in the course of ages, mankind has gradually attained to a partial insight into the relative dependence of phenomena. My duty is to depict the results of our knowledge in all their bearings with reference to the present. In all that is subject to motion and change in space, the ultimate aim, the very expression of physical laws, depend upon 'mean numerical values', which show us the constant amid change, and the stable amid apparent fluctuations of phenomena. Thus the progress of modern physical science is especially characterized by the attainment and the rectification of the mean values of certain quantities by means of the processes of weighing and measuring; and it may be said, that the only remaining and widely-diffused hieroglyphic characters still in our writing — 'numbers' — appear to us again, as powers of the Cosmos, although in a wider sense than that applied to them by the Italian School.

The earnest investigator delights in the simplicity of numerical relations, indicating the dimensions of the celestial regions, the magnitudes and periodical disturbances of the heavenly bodies, the triple elements of terrestrial magnetism, the mean pressure of the atmosphere, and the quantity of heat which the sun imparts in each year, and in every season of the year, to all points of the solid and liquid surface of our planet. These sources of enjoyment do not, however, satisfy the poet of Nature, or the mind of the inquiring many. To both of these the present state of science appears as a blank, now that she answers doubtingly, or wholly rejects as unanswerable, questions to which former ages deemed they could furnish satisfactory replies. In her severer aspect, and clothed with less luxuriance, she shows herself deprived of that seductive charm with which a dogmatizing and symbolizing physical philosophy knew how to deceive the understanding and give the rein to imagination. Long before the discovery of the New World, it was believed that new lands in the Far West might be seen from the shores of the Canaries and the Azores. These illusive images were owing, not to any extraordinary refraction of the rays of light, but produced by an eager longing for the distant and the unattained. The philosophy of the Greeks, the physical views of the Middle Ages, and even those of a more recent period, have been eminently imbued with the charm springing from similar illusive phantoms of the imagination. At the limits of circumscribed knowledge, as from some lofty island shore, the eye delights to penetrate p 82 to distant regions. The belief in the uncommon and the wonderful lends a definite outline to every manifestation of ideal creation; and the realm of fancy — a fairy-land of cosmological, geognostical, and magnetic visions — becomes thus involuntarily blended with the domain of reality.

Nature, in the manifold signification of the word — whether considered as the universality of all that is and ever will be — as the inner moving force of all phenomena, or as their mysterious prototype — reveals itself to the simple mind and feelings of man as something earthly, and closely allied to himself. It is only within the animated circles of organic structure that we feel ourselves peculiarly at home. Thus, wherever the earth unfolds her fruits and flowers, and gives food to countless tribes of animals, there the image of nature impresses itself most vividly upon our senses. The impression thus produced upon our minds limits itself almost exclusively to the reflection of the earthly. The starry vault and the wide expanse of the heavens belong to a picture of the universe, in which the magnitude of masses, the number of congregated suns and faintly glimmering nebulae, although they excite our wonder and astonishment, manifest themselves to us in apparent isolation, and as utterly devoid of all evidence of their being the scenes of organic life. Thus, even in the earliest physical views of mankind, heaven and earth have been separated and opposed to one another as an upper and lower portion of space. If, then, a picture of nature were to correspond to the requirements of contemplation by the senses, it ought to begin with a delineation of our native earth. It should depict, first, the terrestrial planet as to its size and form; its increasing density and heat at increasing depths in its superimposed solid and liquid strate; the separation of sea and land, and the vital forms animating both, developed in the cellular tissues of plants and animals; the atmospheric ocean, with its waves and currents, through which pierce the forest-crowned summits of our mountain chains. After this delineation of purely telluric relations, the eye would rise to the celestial regions, and the Earth would then, as the well-known seat of organic development, be considered as a planet, occupying a place in the series of those heavenly bodies which circle round one of the innumerable host of self-luminous stars. This succession of ideas indicates the course pursued in the earliest stages of perceptive contemplation, and reminds us of the ancient conception of the "sea-girt disk of earth," supporting the vault of heaven. It begins to exercise in action p 83 at the spot where it originated, and passes from the consideration of the known to the unknown, of the near to the distant. It corresponds with the method pursued in our elementary works on astronomy (and which is so admirable in a mathematical point of view), of proceeding from the apparent to the real movements of the heavenly bodies.

Another course of ideas must, however, be pursued in a work which proposes merely to give an exposition of what is known — of what may in the present state of our knowledge be regarded as certain, or as merely probable in a greater or lesser degree — and does not enter into a consideration of the proofs on which such results have been based. Here, therefore, we do not proceed from the subjective point of view of human interests. The terrestrial must be treated only as grand and free, uninfluenced by motives of proximity, social sympathy, or relative utility. A physical cosmography — a picture of the universe — does not begin, therefore, with the picture of the universe — does not begin, therefore, with the terrestrial, but with that which fills the regions of space. But as the sphere of contemplation contracts in dimension our perception of the richness of individual parts, the fullness of physical phenomena, and of the heterogeneous properties of matter becomes enlarged. From the regions in which we recognize ony the dominion of the laws of attraction, we descend to our own planet, and to the intricate play of terrestrial forces. The method here described for the delineation of nature is opposed to that which mst be pursued in establishing conclusive results. The one enumerates what the other demonstrates.

Man learns to know the external world through the organs of the senses. Phenomena of light proclaim the existence of matter in remotest space, and the eye is thus made the medium through which we may contemplate the universe. The discovery of telescopic vision more than two centuries ago, has transmitted to latest generations a power whose limits are as yet unattained.

The first and most general consideration of the Cosmos is that of the 'contents of space' — the distribution of matter, or of creation, as we are wont to designate the assemblage of all that is and ever will be developed. We see matter either agglomerated into rotating, revolving spheres of different density and size, or scattered through space in the form of self-luminous vapor. If we consider first the cosmical vapor dispersed in definite nebulous spots, its state of aggregation will p 84 appear constantly to vary, sometimes appearing separated into round or elliptical disks, single or in pairs, occasionally connected by a thread of light; while, at another time, these nebulae occur in forms of larger dimensions, and are either elongated, or variously branched or fan-shaped or appear like well-defined rings, including a dark interior. It is conjectured that these bodies are undergoing variously developed formative processes, as the cosmical vapor becomes condensed in conformity with the laws of attraction, either round one or more of the nuclei. Between two and three thousand of such unresolvable nebulae, in which the most powerful telescopes have hitherto been unable to distinguish the presence of stars, have been counted, and their positions determined.

The genetic evolution — that perpetual state of development which seems to affect this portion of the regions of space — has led philosophical observers to the discovery of the analogy existing among organic phenomena. As in our forests we see the same kind of tree in all the various stages of its growth, and are thus enabled to form an idea of progressive, vital development, so do we also in the great garden of the universe, recognise the most different phases of sidereal formation. The process of condensation, which formed a part of the doctrines of Anaximenes and of the Ionian School, appears to be going on before our eyes. This subject of investigation and conjecture is especially attractive to the imagination, for in the study of the animated circles of nature, and of the action of all the moving forces of the universe, the charm that exercises the most powerful influence on the mind is derived less from a knowledge of that which 'is' than from a perception of that which 'will be', even though the latter be nothing more than a new condition of a known material existence; for of actual creation, of origin, the beginning of existence from non-existence, we have no experience, and can therefore form no conception.

A comparison of the various causes influencing the development manifested by the greater or less degree of condensation in the interior of nebulae, no less than a successive course of direct observations, have led to the belief that changes of form have been recognized first in Andromeda, next in the constallation Argo, and in the isolated filamentous portion of the nebula in Orion. But want of uniformity in the power of the instruments employed, different conditions of our atmosphere, and other optical relations, render a part of the results invalid as historical evidence.

p 85 'Nebulous stars' must not be confounded either with irregularly-shaped nebulous spots, properly so called, whose separate parts have an unequal degree of brightness (and which may, perhaps, become concentrated into stars as their circumference contracts), nor with the so-called planetary nebulae, whose circular or slightly oval disks manifest in all their parts a perfectly uniform degree of faint light. 'Nebulous stars' are not merely accidental bodies projected upon a nebulous ground, but are a part of the nebulous matter constituting one mass with the body which it surrounds. The not unfrequently considerable magnitude of their apparent diameter, and the remote distance from which they are revealed to us, show that both the planetary nebulae and the nebulous stars must be of enormous dimensions. New and ingenious considerations of the different influence exercised by distance* on the intensity of light of a disk of appreciable diameter, and of a single self-luminous point, render it not improbable that the planetary nebulae are very remote nebulous stars, in which the difference between the central body and the surrounding nebulous covering can no longer be detected by our telescopic instruments.

[footnote] * The optical considerations relative to the difference presented by a single luminous point, and by a disk subtending an appreciable angle, in which the intensity of light is constant at every distance, are explained in Arago's 'Analyse des Travaux de Sir William Herschel' ('Annuaire du Bureau des Long.', 1842, p. 410-412, and 441).

The magnificent zones of the southern heavens, between 50 degrees and 80 degrees, are especially rich in nebulous stars, and in compressed unresolvable nebua e. The larger of the two Magellanic clouds, which circle round the starless, desert pole of the south, appears, according to the most recent researches,* as "a collection of clusters of stars, composed of globular clusters and nebulae of different magnitude, and of large nebulous spots

p 86 not resolvable, which, producing a general brightness in the field of view, form, as it were, the back-ground of the picture."

[footnote] *The two Magellanic clouds, Nubecula major and Nubecula minor, are very remarkable objects. The larger of the two is an accumulated mass of stars, and consists of clusters of stars of irregular form, either conical masses or nebulae of different magnitudes and degrees of condensation. This is interspersed with nebulous spots, not resolvable into stars, but which are probably 'star dust', appearing only as a general radiance upon the telescopic field of a twenty-feet reflector, and forming a luminous ground on which other objects of striking and indescribable form are scattered. In no other portion of the heavens are so many nebulous and stellar masses thronged together in an equally small space. Nubecula minor is much less beautiful, has more unresolvable nebulous light, while the stellar masses are fewer and fainter in intensity. — (From a letter of Sir John Herschel, Feldhuysen, Cape of Good Hope, 13th June, 1836.)

The appearance of these clouds, of the brightly-beaming constellation Argo, of the Milky Way between Scorpio, the Centaur, and the Southern Cross, the picturesque beauty, if one may so speak, of the whole expanse of the southern celestial hemisphere, has left upon my mind an ineffaceable impression. The zodiacal light, which rises in a pyramidal form, and constantly contributes, by its mild radiance, to the external beauty of the tropical nights, is either a vast nebulous ring, rotating between the Earth and Mars, or, less probably, the exterior stratum of the solar atmosphere. Besides these luminous clouds and nebulae of definite form, exact and corresponding observations indicate the existence and the general distribution of an apparently non-luminous, infinitely-divided matter, which posssesses a force of resistance and manifests its presence in Encke's, and perhaps also in Biela's comet, by diminishing their eccentricity and shortening their period of revolution. Of this impending, ethereal, and cosmical matter, it may be supposed that it is in motion; that it gravitates, notwithstanding its original tenuity; that it is condensed in the vicinity of the great mass of the Sun; and, finally, that it may, for myriads of ages, have been augmented by the vapor emanating from the tails of comets.

If we now pass from the consideration of the vaporous matter of the immeasurable regions of space [(Greek)*] — whether scattered without definite form and limits, it exists as a cosmical other, or is condensed into nebulous spots, and becomes comprised among the solid agglomerated bodies of the universe — we approach a class of phenomena exclusively designated by the form of stars, or as the sidereal world.

[footnote] *I should have made use, in the place of garden of the universe, of the beautiful expression [Greek], borrowed by Hesychius from an unknown poet, if [Greek] had not rather signified in general an inclosed space. The connection with the German 'garten' and the English 'garden', 'gards' in Gothic (derived according to Jacob Grimm, from 'gairdan', 'to gird'), is, however, evident, as is likewise the affinity with the Slavonic 'grad', 'gorod', and as Pott remarks, in his 'Etymol. Forschungen', th. i., s. 144 (Etymol. Researches), with the Latin 'chors', whence we have the Spanish 'corte', the French 'cour', and the English word 'court', together with the Ossetic 'khart'. To these may be further added the Scandinavian 'gard',** 'gard', a place inclosed, as a court, or a country seat, and the Persian 'gerd', 'gird', a district, a circle, a princely country seat, a castle or city, as we find the term applied to the names of places in Firdusi's Schahnameh, as 'Siyawakschgird', 'Darabgird', etc.

** (This word is written 'gaard' in the Danish) — Tr.

p 87 Here, too, we find differences existing in the solidity or density of the spheroidally agglomerated matter. Our own solar system presents all stages of 'mean' density (or of the relation of 'volume' to 'mass'.) On comparing the planets from Mercury to Mars with the Sun and with Jupiter, and these two last named with the yet inferior density of Saturn, we arrive, by a descending scale — to draw our illustration from the terrestrial substances — at the respective densities of antimony, honey, water, and pine wood. In comets, which actually constitute the most considerable portion of our solar system with respect to the number of individual forms, the concentrated part, usually termed the 'head', or 'nucleus', transmits sidereal light unimpaired. The mass of a comet probably in no case equals the five thousandth part of that of the earth, so dissimilar are the formative processes manifested in the original and perhaps still progressive agglomerations of matter. In proceeding from general to special considerations, it was particularly desirable to draw attention to this diversity, not merely as a possible, but as an actually proved fact.

The purely speculative conclusions arrived at by Wright, Kant, and Lambert, concerning the general structural arrangement of the universe, and of the distribution of matter in space, have been confirmed by Sir William Herschel, on the more certain path of observation and measurement. That great and enthusiastic, although cautious observer, was the first to sound the depths of heaven in order to determine the limits and form of the starry stratum which we inhabit, and he, too, was the first who ventured to throw the light of investigation upon the relations existing between the position and distance of remote nebulae and our own portion of the sidereal universe. William Herschel, as is well expressed in the elegant inscription on his monument at Upton, broke through the inclosures of heaven ('caelorum perrupit claustra'), and, like another Columbus, penetrated into an unknown ocean, from which he beheld coasts and groups of islands, whose true position it remains for future ages to determine.

Considerations regarding the different intensity of light in stars, and their relative number, that is to say, their numerical frequency on telescopic fields of equal magnitude, have led to the assumption of unequal distances and distribution in space in the strata which they compose. Such assumptions, in as far as they may lead us to draw the limits of the individual portions of the universe, can not offer the same degree of mathematical certainty as that which may be attained in all that p 88 relates to our solar system, whether we consider the rotation of double stars with unequal velocity round one common center of gravity, or the apparent or true movements of all the heavenly bodies. If we take up the physical description of the universe from the remotest nebulae, we may be inclined to compare it with the mythical portions of history. The one begins in the obscurity of antiquity, the other in that of inaccessible space; and at the point where reality seems to flee before us, imagination becomes doubly incited to draw from its own fullness, and give definite outline and permanence to the changing forms of objects.

If we compare the regions of the universe with one of the island-studded seas of our own planet, we may imagine matter to be distributed in groups, either as unresolvable nebulae of different ages, condensed around one or more nuclei, or as already agglomerated into clusters of stars, or isolated spheroidal bodies. The cluster of stars, to which our cosmical island belongs, forms a lens-shaped, flattened stratum, detached on every side, whose major axis is estimated at seven or eight hundred, and its minor one at a hundred and fifty times the distance of Sirius. It would appear, on the supposition that the parallax of Sirius is not greater than that accurately determined for the brightest star in the Centaur (0".9128), that light traverses one distance of Sirius in three years, while it also follows, from Bessel's earlier excellent Memoir* on the parallax of the remarkable star 61 Cygni (0".3483), (whose considerable motion might lead to the inference of great proximity), that a period of nine years and a quarter is required for the transmission of light from this star to our planet.

[footnote] *See Maclear's "Results from 1839 to 1840," in the 'Trans. of the Astronomical Soc.', vol. xii., p. 370, on 'a' Centauri, the probable mean error being 0".0649. For 61 Cygni, see Bessel, in Schumacher's 'Jahrbuch', 1839, s. 47, and Schumacher's 'Astron. Nachr.', bd. xviii., s. 401, 402, probable mean error, 0".0141. With reference to the relative distances of stars of different magnitudes, how those of the third magnitude may probably be three times more remote, and the manner in which we represent to ourselves the material arrangement of the starry strata, I have found the following remarkable passage in Kepler's 'Epitome Astronomiae Copernicanae', 1618, t. i., lib. 1, p. 34-39: "Sol hic noster nil aliud est quam una ex fixis, nobis major et clarior visa, quia propior quam fixa. Pone terram stare ad latus, una semi-diametro via e lactea e, tunc ha ec via lactea apparebit circulus parvus, vel ellipsis parva, tota declinans ad latus alterum; eritque simul uno intuitu conspicua, quae nunc no potest nisi dimidia conspici quovis momento. Itaque fix arum spha era non tantum orbe stellarum, sed etiam circulo lactis versus not deorsum est terminata."

Our starry stratum is a disk of inconsiderable thickness, divided a p 89 third of its length into two branches; it is supposed that we are near this division, and nearer to the region of Sirius than to the constellation Aquila, almost in the middle of the stratum in the line of its thickness or minor axis.

This position of our solar system, and the form of the whole discoidal stratum, have been inferred from sidereal scales, that is to say, from that method of counting the stars to which I have already alluded, and which is based upon the equidistant subdivision of the telescopic field of view. The relative depth of the stratum in all directions is measured by the greater or smaller number of stars appearing in each division. These divisions give the length of the ray of vision in the same manner as we measure the depth to which the plummet has been thrown, before it reaches the bottom, although in the case of a starry stratum there can not, correctly speaking, be any idea of depth, but merely of outer limits. In the direction of the longer axis, where the stars lie behind one another, the more remote ones appear closely crowded together, united, as it were, by a milky-white radiance or luminous vapor, and are perspectively grouped, encircling as in a zone, the visible vault of heaven. This narrow and branched girdle, studded with a radiant light, and here and there interrupted by dark spots, deviates only by a few degrees from forming a perfect large circle round the concave sphere of heaven, owing to our being near the center of the large starry cluster, and almost on the plane of the Milky Way. If our planetary system were far 'outside' this cluster, the Milky Way would appear to telescopic vision as a ring, and at a still greater distance as a resolvable discoidal nebula.

Among the many self-luminous moving suns, erroneously called 'fixed stars', which constitute our cosmical island, our own sun is the only one known by direct observation to be a 'central body' in its relations to spherical agglomerations of matter directly depending upon and revolving round it, either in the form of planets, comets, or aerolite asteroids. As far as we have hitherto been able to investigate 'multiple' stars (double stars or suns), these bodies are not subject, with respect to relative motion and illumination, to the same planetary dependence that characterizes our own solar system. Two or more self-luminous bodies, whose planets and moon, if such exist, have hitherto escaped our telescopic powers of vision, certainly revolve around one common center of gravity; but this is in a portion of space which is probably occupied merely by unagglomerated matter or cosmical vapor, while in our system p 90 the center of gravity is often comprised within the innermost limits of a 'visible' central body. If, therefore, we regard the Sun and the Earth, or the Earth and the Moon, as double-stars, and the whole of our planetary solar system as a multiple cluster of stars, the analogy thus suggested must be limited to the universality of the laws of attraction in different systems, being alike applicable to the independent processes of light and to the method of illumination.

For the generalization of cosmical views, corresponding with the plan we have proposed to follow in giving a delineation of nature or of the universe, the solar system to which the Earth belongs may be considered in a two-fold relation: first, with respect to the different classes of individually agglomerated matter, and the relative size, conformation, density, and distance of the heavenly bodies of this system; and secondly, with reference to other portions of our starry cluster, and of the changes of position of its central body, the Sun.

The solar system, that is to say, the variously-formed matter circling round the Sun, consists, according to the present state of our knowledge of 'eleven primary planets',* eighteen satellites p 91 or secondary planets, and myriads of comets, three of which, known as the "planetary comets," do not pass beyond the narrow limits of the orbits described by the principal planets.

[footnote] * (Since the publication of Baron Humboldt's work in 1845, several other planets have been discovered, making the number of those belonging to our planetary system 'sixteen' instead of 'eleven'. Of these, Astrea, Hebe, Flora, and Iris are members of the remarkable group of asteroids between Mars and Jupiter. Astrea and Hebe were discovered by Hencke at Driesen, the one in 1846 and the other in 1847; Flora and Iris were both discovered in 1847 by Mr. Hind, at the South Villa Observatory, Regent's Park. It would appear from the latest determinations of their elements, that the small planets have the following order with respect to mean distance from the Sun: Flora, Iris, Vesta, Hebe, Astrea, Juno, Ceres, Pallas. Of these, Flora has the shortest period (about 3 1/4 years). The planet Neptune, which, after having been predicted by several astronomers, was actually observed on the 25th of September, 1846, is situated on the confines of our planetary system beyond Uranus. The discovery of this planet is not only highly interesting from the importance attached to it as a question of science, but also from the evidence it affords of the care and unremitting labor evinced by modern astronomers in the investigation and comparison of the older calculations, and the ingenious application of the results thus obtained to the observation of new facts. The merit of having paved the way for the discovery of the planet Neptune is due to M. Bouvard, who, in his persevering and assiduous efforts to deduce the entire orbit of Uranus from observations made during the forty years that succeeded the discovery of that planet in 1781, found the results yielded by theory to be at variance with fact, in a degree that had no parallel in the history of astronomy. This startling discrepancy, which seemed only to gain additional weight from every attempt made by M. Bouvard to correct his calculations, led Leverrier, after a careful modification of the tables of Bouvard, to establish the proposition that there was "a formal incompatibility between the observed motions of Uranus and the hypothesis that he was acted on 'only' by the Sun and known planets, according to the law of universal gravitation." Pursuing this idea, Leverrier arrived at the conclusion that the disturbing cause must be a 'planet', and finally, after an amount of labor that seems perfectly overwhelming, he, on the 31st of August, 1846, laid before the French Institute a paper, in which he indicated the exact spot in the heavens where this new planetary body would be found, giving the following data for its various elements: mean distance from the Sun, 36.154 times that of the Earth; period of revolution, 217.387 years; mean long., Jan. 1st, 1847, 318 degrees 47'; mass, 1/9300th; heliocentric long., Jan 1st1847, 326 degrees 32'. Essential difficulties still intervened, however, and as the remoteness of the planet rendered it improbable that its disk would be discernible by any telescopic instrument, no other means remained for detecting the suspected body but its planetary motion, which could only be ascertained by mapping, after every observation, the quarter of the heavens scanned, and by a comparison of the various maps. Fortunately for the verification of Leverrier's predictions, Dr. Bremiker had just completed a map of the precise region in which it was expected the new planet would apper, this being one of a series of maps made for the Academy of Berlin, of the small stars along the entire zodiac. By means of this valuable assistance, Dr. Galle, of the Berlin Observatory, was led, on the 25th of September, 1846, by the discovery of a star of the eighth magnitude, not recorded in Dr. Bremiker's map, to make the first observation of the planet predicted by Leverrier. By a singular coincidence, Mr. Adams, of Cambridge, had predicted the appearance of the planet simultaneously with M. Leverrier; but by the concurrence of several circumstances much to be regretted, the world at large were not made acquainted with Mr. Adams's valuable discovery until subsequently to the period at which Leverrier published his observations. As the data of Leverrier and Adams stand at present, there is a discrepancy between the predicted and the true distance, and in some other elements of the planet; it remains therefore, for these or future astronomers to reconcile theory with fact, or perhaps, as in the case of Uranus, to make the new planet the means of leading to yet greater discoveries. It would appear from the most recent observations, that the mass of Neptune, instead of being, as at first stated, 1/9300th, is only about 1/23000th that of the Sun, while its periodic time is now given with a greater probability at 166 years, and its mean distance from the Sun nearly 30. The planet appears to have a ring, but as yet no accurate observations have been made regarding its system of satellites. See 'Trans. Astron. Soc.', and 'The Planet Neptune', 1848, by J. P. Nicholl.) — Tr.

We may, with no incondsiderable degree of probability, include within the domain of our Sun, in the immediate sphere of its central force, a rotating ring of vaporous matter, lying probably between the orbits of Venus and Mars, but certainly beyond that of the Earth,* which appears to us in p 92 a pyramidal form, and is known as the 'Zodiacal Light'; and a host of very small asteroids, whose orbits either intersect, or very nearly approach, that of our earth, and which present us with the phenomena of aerolites and falling or shooting stars.

[footnote] * "If there should be molecules in the zones diffused by the atmosphere of the Sun of too volatile a nature either to combine with one another or with the planets, we must suppose that they would, in circling round that luminary, present all the appearances of zodiacal light, without opposing any appreciable resistance to the different bodies composing the planetary system, either owing to their extreme rarity, or to the similarity existing between their motion and that of the planets with which they come in contact." — Laplace, 'Expos. du Syst. du Monde' (ed. 5), p. 415.

When we consider the complication of variously-formed bodies which revolve round the Sun in orbits of such dissimilar eccentricity—although we may not be disposed, with the immortal author of the 'Mecanique Celeste', to regard the largr number of comets as nebulous stars, passing from one central system to another,* we yet can not fail to acknowledge that the planetary system, especially so called (that is, the group of heavenly bodies which, together with their satellites, revolve with but slightly eccentric orbits round the Sun), constitutes but a small portion of the whole system with respect to individual numbers, if not to mass.

[footnote] *Laplace, 'Exp. du Syst. du Monde', p. 396, 414.

It has been proposed to consider the telescopic planets, Vesta, Juno, Ceres, and Pallas, with their more closely intersecting, inclined, and eccentric orbits, as a zone of separation, or as a middle group in space; and if this view be adopted, we shall discover that the interior planetary group (consisting of Mercury, Venus, the Earth, and Mars) presents several very striking contrasts* when compared with the exterior group, comprising Jupiter, Saturn, and Uranus.

[footnote] *Littrow, 'Astronomie', 1825, bd.xi., 107. Mädler, 'Astron.', 1841, § 212. Laplace, 'Exp. du Syst. du Monde', p. 210.

The planets nearest the Sun, and consequently included in the inner group, are of more moderate size, denser, rotate more slowly and with nearly equal velocity (their periods of revolution being almost all about 24 hours), are less compressed at the poles, and with the exception of one, are without satellites. The exterior planets, which are further removed from the Sun, are very considerably larger, have a density five times less, more than twice as great a velocity in the period of their rotation round their axes, are more compressed at the poles, and if six satellites may be ascribed to Uranus, have a quantitative preponderance in the number of their attendant moons, which is as seventeen to one.

p 93 Such general considerations regarding certain characteristic properties appertaining to whole groups, can not, however, be applied with equal justice to the individual planets of every group, nor to the relations between the distances of the revolving planets from the central body, and their absolute size, density, period or rotation, eccentricity, and the inclination of their orbits and the axes. We know as yet of no inherent necessity, no mechanical natural law, similar to the one which teaches us that the squares of the periodic times are proportional to the cubes of the major axes, by which the above-named six elements of the planetary bodies and the form of their orbit are made dependent either on one another, or on their mean distance from the Sun. Mars is smaller than the Earth and Venus, although further removed from the Sun than these last-named planets, approaching most nearly in size to Mercury, the nearest planet to the Sun. Saturn is smaller than Jupiter, and yet much larger than Uranus. The zone of the telescopic planets, which have so inconsiderable a volume, immediately procede Jupiter (the greatest in size of any of the planetary bodies), if we consider them with regard to distance from the Sun; and yet the disks of these small asteroids, which scarcely admit of measurement, have an areal surface not much more than half that of France, Madagascar, or Borneo. However striking may be the extremely small density of all the colossal planets, which are furthest removed from the Sun, we are yet unable in this respect to recognize any regular succession.*

[footnote] *See Kepler, on the increasing density and volume of the planets in proportion with their increase of distance from the Sun, which is described as the densest of all the heavenly bodies; in the 'Epitome Astran. Copern. in' vii. 'libros digesta', 1618-1622, p. 420. Leibnitz also inclined to the opinions of Kepler and Otto von Guericke, that the planets increase in volume in proportion to their increase of distance from the Sun. See his letter to the Magdeburg Burgomaster (Mayence, 1671), in Leibnitz, 'Deutschen Schriften, herausg. von Guhrauer', th. i., 264.

Uranus appears to be denser than Saturn, even if we adopt the smaller mass, 1/24605, assumed by Lamont; and, notwithstanding the inconsiderable difference of density observed in the innermost planetary group,* we find both Venus and Mars less dense than the Earth, which lies between them.

[footnote] *On the arrangement of masses, see Encke, in Schum., 'Astr. Nachr', 1843 Nr. 488, 114.

The time of rotation certainly diminishes with increasing solar distance, but yet it is greater in Mars than in the Earth, and in Saturn than in Jupiter. The elliptic p 94 orbits of Juno, Pallas, and Mercury have the greatest degree of eccentricity, and Mars and Venus, which immediately follow each other, have the least. Mercury and Venus exhibit the same contrasts that may be observed in the four smaller planets, or asteroids, whose paths are so closely interwoven.

The eccentriciities of Juno and Pallas are very nearly identical, and reach three times as great as those of Ceres and Vesta. The same may be said of the inclination of the orbits of the planets toward the plane of projection of the ecliptic, or in the position of their axes of rotation with relation to their orbits, a position on which the relations of climate, seasons of the year, and length of the days depend more than on eccentricity. Those planets that have the most elongated elliptic orbits, as Juno, Pallas, and Mercury, have also, although not to the same degree their orbits most strongly inclined toward the ecliptic. Pallas has a comet-like inclination nearly twenty-six times greater than that of Jupiter, while in the little planet Vesta, which is so near Pallas, the angle of inclination scarcely by six times exceeds that of Jupiter. An equally irregular succession is observed in the position of the axes of the few planets (four or five) whose planes of rotation we know with any degree of certainty. It would appear from the position of the satellites of Uranus, two of which, the second and fourth, have been recently observed with certainty, that the axis of this, the outermost of all the planets is scarcely inclined as much as 11 degrees toward the plane of its orbit, while Saturn is placed between this planet, whose axis almost coincides with the plane of its orbit, and Jupiter, whose axis of rotation is nearly perpendicular to it.

In this enumeration of the forms which compose the world in space, we have delineated them as possessing an actual existence, and not as objects of intellectual contemplation, or as mere links of a mental and causal chain of connection. The planetary system, in its relations of absolute size and relative position of the axes, density, time of rotation, and different degrees of eccentricity of the orbits, does not appear to offer to our apprehension any stronger evidence of a natural necessity than the proportion observed in the distribution of land and water on the Earth, the configuration of continents, or the height of mountain chains. In these respects we can discover no common law in the regions of space or in the inequalities of the earth's crust. They are 'facts' in nature that have arisen from the conflict of manifold forces acting under unknown p 95 conditions, although man considers as 'accidental' whatever he is unable to explain in the planetary formation on purely genetic principles. If the planets have been formed out of separate rings of vaporous matter revolving round the Sun, we may conjecture that the different thickness, unequal density, temperature, and electro-magnetic tension of these rings may have given occasion to the most various agglomerations of matter, in the same manner as the amount of tangential velocity and small variations in its direction have produced so great a differencein the forms and inclinations of the elliptic orbits. Attractions of mass and laws of gravitation have no doubt exercised an influence here, no less than in the geognostic relations of the elevations of continents; but we are unable from the present forms to draw any conclusions regarding the series of conditions through which they have passed. Even the so-called law of the distances of the planets from the Sun, the law of progression (which led Kepler to conjecture the existence of a planet supplying the link that was wanting in the chain of connection between Mars and Jupiter), has been found numerically inexact for the distances between Mercury, Venus, and the Earth, and a variance with the conception of a series, owing to the necessity for a supposition in the case of the first member.

The hitherto disscovered principal planets that revolve round our Sun are attended certainly by fourteen, and probably by eighteen secondary planets (moons or satellites). The principal planets are, therefore, themselves the central bodies of subordinate systems. We seem to recognize in the fabric of the universe the same process of arrangement so frequently exhibited in the development of organic life, where we find in the manifold combinations of groups of plants or animals the same typical form repeated in the 'subordinate classes'. The secondary planets or satellites are more frequent in the external region of the planetary system, lying beyond the intersecting orbits of the smaller planets or asteroids; in the inner region none of the planets are attended by satellites, with the exception of the Earth, whose moon is relatively of great magnitude, since its diameter is equal to a fourth of that of the Earth, while the diameter of the largest of all known secondary planets — the sixth satellite of Saturn — is probably about one seventeenth, and the largest of Jupiter's moons, the third, only about one twenty-sixth part that of the primary planet or central body. The planets which are attended by the largest number of satellites are most remote from the Sun, p 96 and are at the same time the largest, most compressed at the poles, and the least dense. According to the most recent measurements of Mädler, Uranus has a greater planetary compression than any other of the planets, viz., 1/9.92d. In our Earth and her moon, whose mean distance from one another amounts to 207,200 miles, we find that the differences of mass* and diameter between the two are much less considerable than are usually observed to exist between the principal planets and their attendant satellites, or between bodies of different orders in the solar system.

[footnote] *If, according to Burckhardt's determination, the Moon's radius be 0.2725 and its volume 1/49.00th, its density will be 0.5596, or nearly five ninths. Compare, also, Wilh. Beer and H. Madler, 'der Mond', 2, 10, and Madler, 'Ast.', 157. The material contents of the Moon are, according to Hansen, nearly 1/34th (and ädler 1/40.6th) that of the Earth, and its mass equal to 1/87.73d that of the Earth. In the largest of Jupiter's moons, the third, the relations of volume to the central body are 1/15370th, and of mass 1/11300th. On the polar flattening of Uranus, see Schum, 'Astron. Nachr.', 1844, No. 493.

While the density of the Moon is five ninths less than that of the Earth, it would appear, if we may sufficiently depend upon the determinations of their magnitudes and masses, that the second of Jupiter's moons is actually denser than that great planet itself. Among the fourteen satellites that have been investigated with any degree of certainty, the system of the seven satellites of Saturn presents an instance of the greatest possible contrast, both in absolute magnitude and in distance from the central body. The sixth of these satellites is probably not much smaller than Mars, while our moon has a diameter which does not amount to more than half that of the latter planet. With respect to volume, the two outer, the sixth and seventh of Saturn's satellites, approach the nearest to the third and brightest of Jupiter's moons. The two innermost of these satellites belong perhaps, together with the remote moons of Uranus to the smallest cosmical bodies of our solar system, being only made visible under favorable circumstances by the most powerful instruments. They were first discovered by the forty-foot telescope of William Herschel in 1789, and were seen again by John Herschel at the Cape of Good Hope, by Vico at Rome, and by Lamont at Munich. Determinations of the 'true' diameter of satellites, made by the measurement of the apparent size of their small disks, are subjected to many optical difficulties; but numerical astronomy, whose task it is to predetermine by calculation the motions of the heavenly bodies as they will appear when viewed from the Earth, is directed almost p 97 exclusively to motion and mass, and but little to volume. The absolute distance of a satellite from its central body is greatest in the case of the outermost or seventh satellite of Saturn, its distance from the body round which it revolves amounting to more than two millions of miles, or ten times as great a distance as that of our moon from the Earth. In the case of Jupiter we find that the outermost or fourth attendant moon is only 1,040,000 miles from that planet, while the distance between Uranus and its sixth satellite (if the latter really exist) amounts to as much as 1,360,000 miles. If we compare, in each of these subordinate systems, the volume of the satellite, we discover the existence of entirely new numerical relations. The distances of the outermost satellites of Uranus, Saturn, and Jupiter are when expressed in semi-diameters of the main planets, as 91, 64, and 27. The outermost satellite of Saturn appears, therefore, to be removed only about one fifteenth further from the center of that planet than our moon is from the Earth. The first or innermost of Saturn's satellites is nearer to its central body than any other of the secondary planets, and presents, moreover, the only instance of a period of revolution of less than twenty-four hours. Its distance from the center of Saturn may, according to Mädler and Wilhelm Beer, be expressed as 2.47 semi-diameters of that planet, or as 80,088 miles. Its distance from the surface of the main planet is therefore 47,480 miles, and from the outer-most edge of the ring only 4916 miles. The traveler may form to himself an estimate of the smallness of this amount by remembering the statement of an enterprising navigator, Captain Beechey, that he had in three years passed over 72,800 miles. If, instead of absolute distances, we take the semi-diameters of the principal planets, we shall find that even the first or nearest of the moons of Jupiter (which is 26,000 miles further removed from the center of that planet than our moon is from that of the Earth) is only six semi-diameters of Jupiter from its center, while our moon is removed from us fully 60 1/3d semi-diameters of the Earth.

In the subordinate systems of satellites, we find that the same laws of gravitation which regulate the revolutions of the principal planets round the Sun likewise govern the mutual relations existing between these planets among one another and with reference to their attendant satellites. The twelve moons of Saturn, Jupiter, and the Earth all most like the primary planets from west to east, and in elliptic orbits, deviating p 98 but little from circles. It is only in the case of one moon, and perhaps in that of the first and innermost of the satellites of Saturn (0.068), that we discover an eccentricity greater than that of Jupiter; according to the very exact observations of Bessel, the eccentricity of the sixth of Saturn's satellites (0.029) exceeds that of the Earth. On the extremest limits of the planetary system, where, at a distance nineteen times greater than that of our Earth, the centripetal force of the Sun is greatly diminished, the satellites of Uranus (which most striking contrasts from the facts observed with regard to other secondary planets. Instead, as in all other satellites, of having their orbits but slightly inclined toward the ecliptic and (not excepting even Saturn's ring, which may be regarded as a fusion of agglomerated satellites) moving from west to east, the satellites of Uranus are almost perpendicular to the ecliptic, and move retrogressively from east to west, as Sir John Herschel has proved by observations continued during many years. If the primary and secondary planets have been formed by the condensation of rotating rings of solar and planetary atmospheric vapor, there must have existed singular causes of retardation or impediment in the vaporous rings revolving round Uranus, by which, under the relations with which we are unacquainted, the revolution of the second and fourth of its satellites was made to assume a direction opposite to that of the rotation of the central planet.

It seems highly probable that the period of rotation of 'all' secondary planets is equal to that of their revolution round the main planet, and therefore that they always present to the latter the same side. Inequalities, occasioned by sight variations in the revolution, give rise to fluctuations of from 6 degrees to 8 degrees, or to an apparent libration in longitude as well as in latitude. Thus, in the case of our moon, we sometimes observe more than the half of its surface, the eastern and northern edges being more visible at one time, and the western or southern at another. By means of this libration* we are enabled to see the annular mountain Malapert (which occasionally conceals the Moon's south pole), the arctic landscape round the crater of Gioja, and the large gray plane near Endymion which exceeds in superficial extent the 'Mare Vaporum'.

[footnote] *Beer and Madler, op. cit., 185, s.208, and § 347, s. 332; and ix their 'Phys. Kenntniss der himml. Korper', s. 4 und 69, Tab. 1 (Physical History of the Heavenly Bodies).

Three sevenths of the Moon's surface are entirely p 99 concealed from our observation, and must always remain so, unless new and unexpected disturbing causes come into play. These cosmical relations involuntarily remind us of nearly similar conditions in the intellectual world, where, in the domain of deep research into the mysteries and the primeval creative forces of nature, there are regions similarly turned away from us, and apparently unattainable, of which only a narrow margin has revealed itself, for thousands of years, to the human mind, appearing, from time to time, either glimmering in true or delusive light. We have hitherto considered the primary planets, their satellites, and the concentric rings which belong to one, at least, of the outermost planets, as products of tangential force, and as closely connected together by mutual attraction; it therefore now only remains for us to speak of the unnumbered host of 'comets' which constitute a portion of the cosmical bodies revolving in independent orbits round the Sun. If we assume an equable distribution of their orbits, and the limits of their perihelia, or greatest proximities to the Sun, and the possibility of their remaining invisible to the inhabitants of the Earth, and base our estimates on the rules of the calculus of probabilities, we shall obtain as the result an amount of myriads perfectly astonishing. Kepler, with his usual animation of expression, said that there were more comets in the regions of space than fishes in the depths of the ocean. As yet, however, there are scarcely one hundred and fifty whose paths have been calculated, if we may assume at six or seven hundred the number of comets whose appearance and passage through known constellations have been ascertained by more or less precise observations. While the so-called classical nations of the West, the Greeks and Romans, although they may occasionally have indicated the position in which a comet first appeared, never afford any information regarding its apparent path, the copious literature of the Chinese (who observed nature carefully, and recorded with accuracy what they saw) contains circumstantial notices of the constellations through which each comet was observed to pass. These notices go back to more than five hundred years before the Christian era, and many of them are still found to be of value in astronomical observations.*

[footnote] *The first comets of whose orbits we have any knowledge, and which were calculated from Chinese observations, are those of 240 (under Gordian II.), 539 (under Justinian), 565, 568, 574, 837, 1337, and 1385. See John Russell Hind, in Schum., 'Astron. Nachr.', 1843, No. 498. While the comet of 837 (which, according to Du Sejour, continued during twenty-four hours within a distance of 2,000,000 miles from the Earth) terrified Louis I. of France to that degree that he busied himself in building churches and founding monastic establishments, in the hope of appeasing the evils threatened by its appearance, the Chinese astronomers made observations on the path of this cosmical body, whose tail extended over a space of 60 degrees, appearing sometimes single and sometimes multiple. The first comet that has been calculated solely from European observations was that of 1456, known as Halley's comet, from the belief long, but erroneously, entertained that the period when it was first observed by that astronomer was its first and only well-attested appearance. See Arago, in the 'Annuaire', 1836, p. 204, and Langier, 'Comptes Rendus des Seances de l'Acad.', 1843, t. xvi., 1006.

p 100 Although comets have a smaller mass than any other cosmical bodies — being, according to our present knowledge, probably not equal to 1/5000th part of the Earth's mass — yet they occupy the largest space, as their tails in several instances extend over many millions of miles. The cone of luminous vapor which radiates from them has been found, in some cases (as in 1680 and 1811), to equal the length of the Earth's distance from the Sun, forming a line that intersects both the orbits of Venus and Mercury. It is even probable that the vapor of the tails of comets mingled with our atmosphere in the years 1819 and 1823.

Comets exhibit such diversities of form, which appear rather to appertain to the individual than the class, that a description of one of these "wandering light-clouds," as they were already called by Xenophanes and Theon of Alexandria, contemporaries of Pappus, can only be applied with caution to another. The faintest telescopic comets are generally devoid of visible tails, and resemble Herschel's nebulous stars. They appear like circular nebulae of faintly-glimmering vapor, with the light concentrted toward the middle. This is the most simple type; but it can not, however, be regarded as rudimentary, since it might equally be the type of an older cosmical body, exhausted by exhalation. In the larger comets we may distinguish both the so-called "head" or "nucleus," and the single or multiple tail, which is characteristically denominated by the Chinese astronomers "the brush" ('sui'). The nucleus generally presents no definite outline, although, in a few rare cases, it appears like a star of the first or second magnitude, and has even been seen in bright sunshine;* as, p 101 for instance, in the large comets of 1402, 1532, 1577, 1744, and 1843.

[footnote] *Arago, 'Annuaire', 1832, p. 209, 211. The phenomenon of the tail of a comet being visible in bright sunshine, which is recorded of the comet of 1402, occurred again in the case of the large comet of 1843, whose nucleus and tail were seen in North America on the 28th of February (according to the testimony of J. G. Clarke, of Portland, state of Maine), between 1 and 3 o'clock in the afternoon.(a) The distance of the very dense nucleus from the sun's light admitted of being measured with much exactness. The nucleus and tail appeared like a very pure white cloud, a darker space intervening between the tail and the nucleus. ('Amer. Journ. of Science', vol. xiv., No. 1, p. 229.)

[footnote] (a) [The translator was at New Bedford, Massachusetts, U.S., on the 28th February, 1843, and distinctly saw the comet, between 1 and 2 in the afternoon. The sky at the time was intensely blue, and the sun shining with a dazzling brightness unknown in European climates.] — Tr

This latter circumstance indicates, in particular individuals, a denser mass, capable of reflecting light with greater intensity. Even in Herschel's large telescope, only two comets, that discovered in Sicily in 1807, and the splendid one of 1811, exhibited well-defined disks;* the one at an angle of 1 second, and the other at 0.77 seconds, whence the true diameters are assumed to be 536 and 428 miles.

[footnote] *'Phil. Trans.' for 1808, Part ii., p. 155, and for 1812, Part i., p. 118. The diameters found by Herschel for the nuclei were 538 and 428 English miles. For the magnitudes of the comets of 1798 and 1805, see Arago, 'Annuaire', 1832, p. 203.

The diameters of the less well-defined nuclei of the comets of 1798 and 1805 did not appear to exceed 24 or 28 miles.

In several comets that have been investigated with great care, especially in the above-named one of 1811, which continued visible for so long a period, the nucleus and its nebulous envelope were entirely separated from the tail by a darker space. The intensity of light in the nucleus of comets does not augment toward the center in any uniform degree, brightly shining zones being in many cases separated by concentric nebulous envelopes. The tails sometimes appear single, sometimes, although more rarely, double; and in the comets of 1807 and 1843 the branches were of different lengths; in one instance (1744) the tail had six branches, the whole forming an angle of 60 degrees. The tails have been sometimes straight, sometimes curved, either toward both sides, or toward the side appearing to us as the exterior (as in 1811), or convex toward the direction in which the comet is moving (as in that of 1618); and sometimes the tail has even appeared like a flame in motion. The tails are always turned away from the sun, so that their line of prolongation passes through its center; a fact which, according to Edward Biot, was noticed by the Chinese astronomers as early as 837, but was first generally made known in Europe by Fracastoro and Peter Apian in the sixteenth century. These emanations may be regarded as conoidal envelopes of greater of less thickness, p 102 and, considered in this manner, they furnish a simple explanation of many of the remarkable optical phenomena already spoken of.

Comets are not only characteristically different in form, some being entirely without a visible tail, while others have a tail of immense length (as in the instance of the comet of 1618, whose tail measured 104 degrees), but we also see the same comets undergoing successive and rapidly-changing processes of configuration. These variations of form have been most accurately and admirably described in the comet of 1744, by Hensius, at St. Petersburg, and in Halley's comet, on its last reappearance in 1835, by Bessel, at Konigsberg. A more or less well-defined tuft of rays emanated from that part of the nucleus which was turned toward the Sun; and the rays being bent backward, formed a part of the tail. The nucleus of Halley's comet; with its emanations, presented the appearance of a burning rocket, the end of which was turned sideways by the force of the wind. The rays issuing from the head were seen by Arago and myself, at the Observatory at Paris, to assume very different forms on successive nights.*

[footnote] *Arago, 'Des Changements physiques de la Comete de Halley du 15-23 Oct., 1835. 'Annuaire', 1836, p. 218, 221. The ordinary direction of the emanations was noticed even in Nero's time. "Comae radios solis effugiunt." — Seneca, 'Nat. Quaest.', vii., 20.

The great Konigsberg astronomer concluded from many measurements, and from theoretical considerations, "that the cone of light issuing from the comet deviated considerably both to the right and the left of the true direction of the Sun, but that it always returned to that direction, and passed over to the opposite side, so that both the cone of light and the body of the comet from whence it emanated experienced a rotatory, or, rather, a vibratory motion in the plane of the orbit." He finds that "the attractive force exercised by the Sun on heavy bodies is inadequate to explain such vibrations, and is of opinion that they indicate a polar force, which turns one semi-diameter of the comet toward the Sun, and strives to turn the opposite side away from that luminary. The magnetic polarity possessed by the Earth may present some analogy to this, and, should the Sun have an opposite polarity, an influence might be manifested, resulting in the precession of the equinoxes." This is not the place to enter more fully upon the grounds on which explanations of this subject have been based; but observations so remarkable,* and views of so exalted p 103 a character, regarding the most wonderful class of the cosmical bodies belonging to our solar system, ought not to be entirely passed over in this sketch of a general picture of nature.

[footnote] *Bessel, in Schumacher, 'Astr. Nachr.', 1836, No. 300-302, s. 188, 192, 197, 200, 202, und 230. Also in Schumacher, 'Jahrb.', 1837, s. 149, 168. William Herschel, in his observations on the beautiful comet of 1811, believed that he had discovered evidences of the rotation of the nucleus and tail ('Phil. Trans.' for 1812, Part i., p. 140). Dunlop, at Paramatta thought the same with reference to the third comet of 1825.

Although, as a rule, the tails of comets increase in magnitude and brilliancy in the vicinity of the sun, and are directed away from that central body, yet the comet of 1823 offered the remarkable example of two tails, one of which was turned toward the sun, and the other away from it, forming with each other an angle of 160 degrees. Modifications of polarity and the unequal manner of its distribution, and of the direction in which it is conducted, may in this rare instance have occasioned a double, unchecked, continuous emanation of nebulous matter.*

[footnote] *Bessel, in 'Astr. Nachr.', 1836, No. 302, s. 231. Schum, 'Jahrb.', 1837 s. 175. See, also Lehmann, 'Ueber Cometenschweife' (On the Tails of Comets), in Bode, 'Astron. Jahrb. fur' 1826, s. 168.

Aristotle, in his 'Natural Philosophy', makes these emanations the means of bringing the phenomena of comets into a singular connection with the existence of the Milky Way. According to his views, the innumerable quantity of stars which compose this starry zone give out a self-luminous, incandescent matter. The nebulous belt which separates the different portions of the vault of heaven was therefore regarded by the Stagirite as a large comet, the substance of which was incessantly being renewed.*

[footnote] *Aristot., 'Meteor.', i., 8, 11-14, und 19-21 (ed. Ideler, t. i., p. 32-34). Biese, 'Phil. des Aristoteles', bd. ii., s. 86. Since Aristotle exercised so great an influence throughout the whole of the Middle Ages, it is very much to be regretted that he was so averse to those grander views of the elder Pythagoreans, which inculcated ideas so nearly approximating to truth respecting the structure of the universe. He asserts that comets are transitory meteors belonging to our atmosphere in the very book in which he cites the opinion of the Pythagorean school, according to which these cosmical bodies are supposed to be planets having long periods of revolution. (Aristot., i., 6, 2.) This Pythagorean doctrine, which, according to the testimony of Apollonius Myndius, was still more ancient, having originated with the Chaldeans, passed over to the Romans, who in this instance, as was their usual practice, were merely the copiers of others. The Myndian philosopher describes the path of comets as directed toward the upper and remote regions of heaven. Hence Seneca says, in his 'Nat. Quaest.', vii., 17: "Cometes non est species falsa, sed proprium sidus sicut solis et lunae: altiora mundi secat et tunc demum apparet quum in imum cursum sui venit;" and again (at vii., 27), "Cometes aternos esse et sortis ejusdem, cujus caetera (sidera), etiamsi faciem illis non habent similem." Pliny (ii., 25) also refers to Apollonius Myndius, when he says, "Sunt qui et haec sidera perpetua esse credant suoque ambitu ire, sed non nisi relicta a sole cerni."

p 104 The occulation of the fixed stars by the nucleus of a comet, or by its innermost vaporous envelopes, might throw some light on the physical character of these wonderful bodies; but we are unfortunately deficient in observations by which we may be assured* that the occulation was perfectly central; for, as it has already been observed, the parts of the envelope contiguous to the nucleus are alternately composed of layers of dense or very attenuated vapor.

[footnote] *Olbers, in 'Astr. Nachr.', 1828, s. 157, 184. Arago, 'De la Constitution physique des Cometes; Annuaire de' 1832, p. 203, 208. The ancients were struck by the phenomenon that it was possible to see through comets as through a flame. The earliest evidence to be met with of stars having been seen through comets is that of Democritus (Aristot., 'Meteor.', i., 6, 11), and the statement leads Aristotle to make the not unimportant remark, that he himself had observed the occulation of one of the stars of Gemini by Jupiter. Seneca only speaks decidedly of the transparence of the tail of comets. "We may see," says he, "stars through a comet as through a cloud ('Nat. Quaest.', vii., 18); but we can ony see through the rays of the tail, and not through the body of the comet itself: 'non in ea parte qua sidus ipsum est spissi et solidi ignis, sed qua rarus splendor occurrit et in crines dispergitur. Per intervalla ignium, non er ipsos, vides" (vii., 26). The last remark is unnecessary, since, as Galileo observed in the 'Saggiatore (Lettera a Monsignor Cesarini', 1619), we can certainly see through a flame when it is not of too great a thickness'.

On the other hand the carefully conducted measurements of Bessel prove, beyond all doubt, that on the 29th of September, 1835, the light of a star of the tenth magnitude, which was then at a distance of 7".78 from the central point of the head of Halley's comet, passed through very dense nebulous matter, without experiencing any deflection during its passage.*

[footnote] *Bessel, in the 'Astron. Nachr.', 1836, No. 301, s. 204, 206. Struve, in 'Recueil des Mem. de l'Acad. de St. Peterab.', 1836, p. 140, 143, and 'Astr. Nachr.', 1836, No. 303, s. 238, writes as follows: "At Dorpat the star was in conjunction only 2".2 from the brightest point of the comet. The star remained continually visible, and its light was not perceptibly diminished, while the nucleus of the comet seemed to be almost extinguished before the radiance of the small star of the ninth or tenth magnitude."

If such an absence of refracting power must be ascribed to the nucleus of a comet, we can scarcely regard the matter composing comets as a gaseous fluid. The question here arises whether this absence of refracting power may not be owing to the extreme tenuity of the fluid; or does the comet consist of separated particles, constituting a cosmical stratum of clouds, which, like the clouds of our atmosphere, that exercise no influence on the p 105 zenith distance of the stars, does not affect the ray of light passing through it? In the passage of a comet over a star, a more or less considerable diminution of light has often been observed; but this has been justly ascribed to the brightness of the ground from which the star seems to stand forth during the passage of the comet.

The most important and decisive observations that we possess on the nature and the light of comets are due to Arago's polarization experiments. His polariscope instructs us regarding the physical constitution of the Sun and comets, indicating whether a ray that reaches us from a distance of many millions of miles transmits light directly or by reflection; and if the former, whther the source of light is a solid, a liquid, or a gaseous body. His apparatus was used at the Paris Observatory in examining the light of Capella and that of the great comet of 1819. The latter showed polarized, and therefore reflected light, while the fixed star, as was to be expected, appeared to be a self-luminous sun.*

[footnote] *On the 3d of July, 1819, Arago made the first attempt to analyze the light of comets by polarization, on the evening of the sudden appearance of the great comet. I was present at the Paris Observatory, and was fully convinced, as were also Matthieu and the late Bouvard of the dissimilarity in the intensity of the light seen in the polariscope, when the instrument received cometary light. When it received light from Capella, which was near the comet, and at an equal altitude, the images were of equal intensity. On the reappearance of Halley's comet in 1835, the instrument was altered so as to give, according to Arago's chromatic polarization, two images of complementary colors (green and red). ('Annales de Chimie', t. xiii., p. 108; 'Annuaire', 1832, p. 216.) "We must conclude from these observations," says Arago, "that the cometary light was not entirely composed of rays having the properties of direct light, there being light which was reflected specularly or polarized, that is, coming from the sun. It can not be stated with absolute certainty that comets shine only with borrowed light, for bodies, in becoming self-luminous, do not, on that account, lose the power of reflecting foreign light."

The existance of polarized cometary light announced itself not only by the inequality of the images, but was proved with greater certainty on the reappearance of Halley's comet, in the year 1835, by the more striking contrast of the complementary colors, deduced from the laws of chromatic polarization discovered by Arago in 1811. These beautiful experiments still leave it undecided whether, in addition to this reflected solar light, comets may not have light of their own. Even in the case of the planets, as, for instance, in Venus, an evolution of independent light seems very probable.

The variable intensity of light in comets can not always be p 106 explained by the position of their orbits and their distance from the Sun. It would seem to indicate, in some individuals, the existence of an inherent process of condensation, and an increased or diminished capacity of reflecting borrowed light. In the comet of 1618, and in that which has a period of three years, it was observed first by Hevelius that the nucleus of the comet diminished at its perihelion and enlarged at its aphelion, a fact which, after remaining long unheeded, was again noticed by the talented astronomer Valz at Nismes. The regularity of the change of volume, according to the different degrees of distance from the Sun, appears very striking. The physical explanation of the phenomenon can not, however, be sought in the condensed layers of cosmical vapor occurring in the vicinity of the Sun, since it is difficult to imagine the nebulous envelope of the nucleus of the comet to be vesicular and impervious to the other.*

[footnote] *Arago, in the 'Annuaire', 1832, p. 217-220. Sir John Herschel, 'Astron.', 488.

The dissimilar eccentricity of the orbits of comets has, in recent times (1819), in the most brilliant manner enriched our knowledge of the solar system. Encke has discovered the existence of a comet of so short a period of revolution that it remains entirely within the limits of our planetary system, attaining its aphelion between the orbits of the smaller planets and that of Jupiter. Its eccentricity must be assumed at 0.845, that of Juno (which has the greatest eccentricity of any of the planets) being 0.255. Encke's comet has several times, although with difficulty, been observed by the naked eye, as in Europe in 1819, and according to Rumker, in New Holland in 1822. Its period of revolution is about 3 1/3d years; but, from a careful comparison of the epochs of its return to its perihelion, the remarkable fact has been discovered that these periods have diminished in the most regular manner between the years 1786 and 1838, the diminution amounting, in the course of 52 years, to about 1 3/10th days. The attempt to bring into unison the results of observation and calculation in the investigation of all the planetary disturbances, with the view of explaining this phenomenon, has led to the adoption of the very probable hypothesis that there exists dispersed in space a vaporous substance capable of acting as a resisting medium. This matter diminished the tangential force, and with it the major axis of the comet's orbit. The value of the constant of the resistance appears to be somewhat different before and after the perihelion; and this may, perhaps, be ascribed p 107 to the altered form of the small nebulous star in the vicinity of the Sun, and to the action of the unequal density of the strata of cosmical ether.*

[footnote] *Encke, in the 'Astronomiche Nachrichten', 1843, No. 489, s. 130-132.

These facts, and the investigations to which they have led, belong to the most interesting results of modern astronomy. Encke's comet has been the means of leading astronomers to a more exact investigation of Jupiter's mass (a most important point with reference to the calculation of perturbations); and, more recently, the course of this comet has obtained for us the first determination, although only an approximative one, of a smaller mass for Mercury.

The discovery of Encke's comet, which had a period of only 3 1/3d years, was speedily followed, in 1826, by that of another, Biela's comet, whose period of revolution is 6 3/4th years, and which is likewise planetary, having its aphelion beyond the orbit of Jupiter, but within that of Saturn. It has a fainter light than Encke's comet, and, like the latter, its motion is direct, while Halley's comet moves in a course opposite to that pursued by the planets. Biela's comet presents the first certain example of the orbit of a comet intersecting that of the Earth. This position, with reference to our planet, may therefore be productive of danger, if we can associate an idea of danger with so extraordinary a natural phenomenon, whose history presents no parallel, and the results of which we are consequently unable correctly to estimate. Small masses endowed with enormous velocity may certainly exercise a considerable power; but Laplace has shown that the mass of the comet of 1770 is probably not equal to 1/5000th that of the Earth, or about 1/2000th that of the Moon.*

[footnote] *Laplace, 'Expos. du Syst. du Monde', p. 216, 237.

We must not confound the passage of Biela's comet through the Earth's orbit with its proximity to, or collision with our globe. When this passage took place, on the 29th of October, 1832, it required a full month before the Earth would reach the point of intersection of the two orbits. These two comets of short periods of revolution also intersect each other, and it has been justly observed,* that amid the many perturbations experienced by such small bodies from the largr planets, there is a 'possibility' — supposing a meeting of these comets to occur in October — that the inhabitants of the Earth may witness the extraordinary spectacle of an encounter between two cosmical bodies, and possibly of their reciprocal penetration and amalgamation, or of their destruction by means of exhausting emanations.

[footnote] *Littrow, 'Beschreibende Astron.', 1835, s. 274. On the inner comet recently discovered by M. Faye, at the Observatory of Paris, and whose eccentricity is 0.551, its distance at its perihelion 1.690, and its distance at its aphelion 5.832, see Schumacher, 'Astron. Nachr.', 1844, No. 495. Regarding the supposed identity of the comet of 1766 with the third comet of 1819, see 'Astr. Nachr.', 1833, No. 239; and on the identity of the comet of 1743 and the fourth comet of 1819, see No. 237 or the last mentioned work.

Events of this nature, resulting either from deflection occasioned by disturbing masses or primevally intersecting orbits, must have been of frequent occurrence in the course of millions of years in the immeasurable regions of ethereal space; but they must be regarded as isolated occurrences, exercising no more general or alternative effects on cosmical relations than the breaking forth or extinction of a volcano within the limited sphere of our Earth.

A third interior comet, having likewise a short period of revolution was discovered by Faye on the 22d of November, 1843, at the Observatory at Paris. Its elliptic path, which approaches much more nearly to a circle than that of any other known comet, is included within the orbits of Mars and Saturn. This comet, therefore, which, according to Goldschmidt, passes beyond the orbit of Jupiter, is one of the few whose perihelia are beyond Mars. Its period of revolution is 7 29/100 years, and it is not improbable that the form of its present orbit may be owing to its great approximation to Jupiter at the close of the year 1839.

If we consider the comets in their inclosed elliptic orbits as members of our solar system, and with respect to the length of their major axes, the amount of their eccentricity, and their periods of revolution, we shall probably find that the three planetary comets of Encke, Biela, and Faye are most nearly approached in these respects, first, by the comet discovered in 1766 by Messier, and which is regarded by Clausen as identical with the third comet of 1819; and next, by the fourth comet of the last-mentioned year, discovered by Blaupain, but considered by Clausen as identical with that of the year 1743, and whose orbit appears, like that of Lexell's comet, to have suffered great variations from the proximity and attraction of Jupiter. The two last-named comets would likewise seem to have a period of revolution not exceeding five or six years, and their aphelia are in the vicinity of Jupiter's orbit. Among the comets that have a period of revolution of from seventy to p 109 seventy-six years, the first in point of importance with respect to theoretical and physical astronomy is Halley's comet, whose last appearance, in 1835, was much less brilliant than was to be expected from preceding ones; next we would notice Olbers's comet, discovered on the 6th of March, 1815; and, lastly, the comet discovered by Pons in the year 1812, and whose elliptic orbit has been determined by Encke. The two latter comets were invisible to the naked eye. We now know with certainty of nine returns of Halley's large comet, it having recently been proved by Laugier's calculations*, that in the Chinese table of comets, first made known to us by Edward Biot, the comet of 1378 is identical with Halley's; its periods of revolution have varied in the interval between 1378 and 1835 from 74.91 to 77.58 years, the mean being 76.1.

[footnote] *Laugier, in the 'Comptes Rendus des Seances de l'Academie', 1843, t. xvi., p. 1006.

A host of other comets may be contrasted with the cosmical bodies of which we have spoken, requiring several thousand years to perform their orbits, which it is difficult to determine with any degree of certainty. The beautiful comet of 1811 requires, according to Argelander, a period of 3065 years for its revolution, and the colossal one of 1680 as much as 8800 years, according to Encke's calculation. These bodies respectively recede, therefore, 21 and 44 times further than Uranus from the Sun, that is to say, 33,600 and 70,400 millions of miles. At this enormous distance the attractive force of the Sun is still manifested; but while the velocity of the comet of 1680 at its perihelion is 212 miles in a second, that is, thirteen times greater than that of the Earth, it scarcely moves ten feet in the second when at its aphelion. This velocity is only three times greater than that of water in our most sluggish European rivers, and equal only to half that which I have observed in the Cassiquiare, a branch of the Orinoco. It is highly probable that, among the innumerable host of uncalculated or undiscovered comets, there are many whose major axes greatly exceed that of the comet of 1680. In order to form some idea by numbers, I do not say of the sphere of attraction, but of the distance in space of a fixed star, or other sun, from the aphelion of the comet of 1680 (the furthest receding cosmical body with which we are acquainted in our solar system), it must be remembered that, according to the most recent determinations of parallaxes, the nearest fixed star is full 250 times further removed from our sun than the comet in its aphelion. The comet's distance is only 44 p 110 times that of Uranus, while 'a' Centauri is 11,000 and 61 Cygni 31,000 times that of Uranus, according to Bessel's determinations.

Having considered the greatest distances of comets from the central body, it now remains for us to notice instances of the greatest proximity hitherto measured. Lexell and Burckhardt's comet of 1770, so celebrated on account of the disturbances it experienced from Jupiter, has approached the Earth within a smaller distance than any other comet. On the 28th of June, 1770, its distance from the Earth was ony six times than of the Moon. The same comet passed twice, viz., in 1769 and 1779, through the system of Jupiter's four satellites without producing the slightest notable change in the well-known orbits of these bodies. The great comet of 1680 approached at its perihelion eight or nine times nearer to the surface of the Sun than Lexell's comet did to that of our Earth, being on the 17th of December a sixth part of the Sun's diameter, or seven tenths of the distance of the Moon from that luminary. Perihelia occurring beyond the orbit of Mars can seldom be observed by the inhabitants of the Earth, owing to the faintness of the light of distant comets; and among those already calculated the comet of 1729 is the only one which has its perihelion between the orbits of Pallas and Jupiter; it was even observed beyond the latter.

Since scientific knowledge, although frequently blended with vague and superficial views, has been more extensively diffused through wider circles of social life, apprehensions of the possible evils threatened by comets have acquired more weight as their direction has become more definite. The certainty that there are within the known planetary orbits comets which revisit our regions of space at short intervals — that great disturbances have been produced by Jupiter and Saturn in their orbits, by which such as were apparently harmless have been converted into dangerous bodies — the intersection of the Earth's orbit by Biela's comet — the cosmical vapor, which, acting as a resisting and impeding medium, tends to contract all orbits — the individual difference of comets, which would seem to indicate considerable decreasing gradations in the quantity of the mass of the nucleus, are all considerations more than equivalent, both as to number and variety, to the vague fears entertained in early ages of the general conflagration of the world by 'flaming swords', and stars with 'fiery streaming hair'. As the consolatory considerations which may be derived from the calculus of probabilities address themselves to reason and to p 111 meditative understanding only, and not to the imagination or to a desponding condition of mind, modern science has been accused, and not entirely without reason, of not attempting to allay apprehensions which it has been the very means of exciting. It is an inherent attribute of the human mind to experience fear, and not hope or joy, at the aspect of that which is unexpected and extraordinary.*

[footnote] *Fries, 'Vorlesungen uber die Sternkunde', 1833, s. 262-267 (Lectures on the Science of Astronomy). An infelicitously chosen instance of the good omen of a comet may be found in Seneca, 'Nat. Quest.', vii., 17 and 21. The philosopher thus writes of the comet: "Quem nos Neronis principatu latissimo vidimus et qui cometis detraxit infamiam."

The strange form of a large comet, its faint nebulous light, and its sudden appearance in the vault of heaven, have in all regions been almost invariably regarded by the people at large as some new and formidable agent inimical to the existing state of things. The sudden occurrence and short duration of the phenomenon lead to the belief of some equally rapid reflection of its agency in terrestrial matters, whose varied nature renders it easy to find events that may be regarded as the fulfillment of the evil foretold by the appearance of these mysterious cosmical bodies. In our own day, however, the public mind has taken another and more cheerful, although singular, turn with regard to comets; and in the German vineyards in the beautiful valleys of the Rhine and Moselle, a belief has arisen, ascribing to these once ill-omened bodies a beneficial influence on the ripening of the vine. The evidence yielded by experience, of which there is no lack in these days, when comets may so frequently be observed, has not been able to shake the common belief in the meteorological myth of the existence of wandering stars capable of radiating heat.

This material taken from pages 111- 147

COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1 by Alexander von Humboldt

Translated by E C Otte

from the 1858 Harper & Brothers edition of Cosmos, volume 1 —————————————————————————

From comets I would pass to the consideration of a far more enigmatical class of agglomerated matter — the smallest of all asteroids, to which we apply the name 'aërolites', or 'meteoric stones',* when they reach our atmosphere in a fragmentary condition.

[footnote] * (Much valuable information may be obtained regarding the origin and composition of aërolites or meteoric stones in Memoirs on the subject, by Baumbeer and other writers, in the numbers of Poggendorf's 'Annalen', from 1845 to the present time.) — Tr.

If I should seem to dwell on the specific enumeration of these bodies, and of comets, longer than the general nature of this work might warrant, I have not done so undesignedly. The diversity existing in the individual characteristics of comets has already been noticed. The imperfect knowledge we possess of their physical character renders it p 112 diifficult in a work like the present, to give the proper degree of circumstantiality to the phenomena, which, although of frequent recurrence, have been observed with such various degrees of accuracy, or to separate the necessary from the accidental. It is only with respect to measurements and computations that the astronomy of comets has made any marked advancement, and, consequently, a scientific consideration of these bodies must be limited to a specification of the differences of physiognomy and conformation in the nucleus and tail, the instances of great approximation to other cosmical bodies, and of the extremes in the length of their orbits and in their periods of revolution. A faithful delineation of these phenomena, as well as of those which we proceed to consider, can only be given by sketching individual features with the animated circumstantiality of reality.

Shooting stars, fire-balls, and meteoric stones are, with great probability, regarded as small bodies moving with planetary velocity, and revolving in obedience to the laws of general gravity in conic sections round the Sun. When these masses meet the Earth in their course, and are attracted by it, they enter within the limits of our atmosphere in a luminous condition, and frequently let fall more or less strongly heated stony fragments, covered with a shining black crust. When we enter into a careful investigation of the facts observed at those epochs when showers of shooting stars fell periodically in Cumana in 1799, and in North America during the years 1833 and 1834, we shall find that 'fire-balls' can not be considered separately from shooting stars. Both these phenomena are frequently not only simultaneous and blended together, but they likewise are often found to merge into one another, the one phenomenon gradually assuming the character of the other alike with respect to the size of their disks, the emanation of sparks, and the velocities of their motion. Although exploding smoking luminous fire-balls are sometimes seen, even in the brightness of tropical daylight,* equaling in size the apparent p 113 diameter of the Moon, innumerable quantities of shooting stars have, on the other hand, been observed to fall in forms of such extremely small dimensions that they appear only as moving points or 'phosphorescent lines.'**

[footnote] *A friend of mine, much accustomed to exact trigonometrical measurements, was in the year 1788 at Popayan, a city which is 2 degrees 26' north latitude, lying at an elevation of 5583 feet above the level of the sea, and at noon, when the sun was shining brightly in a cloudless sky, saw his room lighted up by a fire-ball. He had his back to the window at the time, and on turning round, perceived that great part of the path traversed by the fire-ball was still illuminated by the brightest radiance. Different nations have had the most various terms to express these phenomena: The Germans use the word 'Sternschnuppe', literally 'star snuff' — an expression well suited to the physical views of the vulgar in former times, according to which, the lights in the firmament were said to undergo a process of 'snuffing' or cleaning; and other nations generally adopt a term expressive of a 'shot' or 'fall' of stars, as the Swedish 'stjernifall', the Italian 'stella cadente', and the English 'star shoot.' In the woody district of the Orinoco, on the dreary banks of the Cassiquiare, I heard the natives in the Mission of Vasiva use terms still more inelegant than the German 'star snuff.' ('Relation Historique du Voy. aux Régions Equinox.', t. ii., p. 513.) These same tribes term the pearly drops of dew which cover the beautiful leaves of the heliconia 'star spit.' In the Lithuanian mythology, the imagination of the people has embodied its ideas of the nature and signification of falling stars under nobler and more graceful symbols. The Parcæ, 'Werpeja', weave in heaven for the new-born child its thread of fate, attaching each separate thread to a star. When death approaches the person, the thread is rent, and the star wanes and sinks to the earth. Jacob Grimm, 'Deutsche Mythologie', 1843, s. 685.

[footnote] ** According to the testimony of Professor Denison Olmsted, of Yale College, New Haven, Connecticut. (See Poggend., 'Annalen der Physik', bd. xxx., s. 194.) Kepler, who excluded fire-balls and shooting stars from the domain of astronomy, because they were, according to his views, "meteors arising from the exhalations of the earth, and blending with the higher ether," expresses himself, however, generally with much caution. He says: "Stellæ cadentes sunt materia viscida inflammata. Earum aliquæ inter cadendum absumuntur, aliquæ verè in terram cadunt, pondere suo tractæ. Nec est dissimile vero, quasdam conglobatas esse ex materia fæculentâ, in ipsam auram ætheream immixta: exque aëtheris regione, tractu rectilineo, per aërem trajicere, ceu minutos competas, occultâ causa motus utrorumque." — Kepler, 'Epit. Astron. Copernicanæ', t. i., p. 80.

It still remains undertermined whether the many luminous bodies that shoot across the sky may not vary in their nature. On my return from the equinoctial zones, I was impressed with an idea that in the torrid regions of the tropics I had more frequently than in our colder latitudes seen shooting stars fall as if from a height of twelve or fifteen thousand feet; that they were of brighter colors, and left a more brilliant line of light in their track; but this impression was no doubt owing to the greater transparency of the tropical atmosphere*, which enables the eye to penetrate further into distance.

[footnote] *'Relation Historique', t. i., p. 80, 213, 527. If in falling stars, as in comets, we distinguish between the head or nucleus and the tail, we shall find that the greater transparency of the atmosphere in tropical climates is evinced in the greater length and brilliancy of the tail which may be observed in those latitudes. The phenomenon is therefore not necessarily more frequent there, because it is oftener seen and continues longer visible. The influence exercised on shooting stars by the character of the atmosphere is shown occasionally even in our temperate zone, and at very small distances apart. Wartmann relates that on the occasion of a November phenomenon at two places lying very near each other, Geneva and Aux Planchettes, the number of the meteors counted were as 1 to 7. (Wartmann, 'Mém. sur les Etoiles filantes', p. 17.) The tail of a shooting star (or its 'train'), on the subject of which Brandes has made so many exact and delicate observations, is in no way to be ascribed to the continuance of the impression produced by light on the retina. It sometimes continues visible a whole minute, and in some rare instances longer than the light of the nucleus of the shooting star; in which case the luminous track remains motionless. (Gilb., 'Ann.', bd. xiv., s. 251.) This circumstance further indicates the analogy between large shooting stars and fire-balls. Admiral Krusenstern saw, in his voyage round the world, the train of a fire-ball shine for an hour after the lluminous body itself had disappeared, and scarcely move throughout the whole time. ('Reise', th. i., s. 58.) Sir Alexander Burnes gives a charming description of the transparency of the clear atmosphere of Bokhara, which was once so favorable to the pursuit of astronomical observations. Bokhara is situated in 39 degrees 48' north latitude, and at an elevation of 1280 feet above the level of the sea. "There is a constant serenity in its atmosphere, and an admirable clearness in the sky. At night, the stars have uncommon luster, and the Milky Way shines gloriously in the firmament. There is also a never-ceasing display of the most brilliant meteors, which dart like rockets in the sky; ten or twelve of them are sometimes seen in an hour, assuming every color — fiery red, blue, pale, and faint. It is a noble country for astronomical science, and great must have been the advantage enjoyed by the famed observatory of Samarkand." (Burnes, 'Travels into Bokhara', vol. ii. (1834), p. 158.) A mere traveler must not be reproached for calling ten or twelve shooting stars in an hour "many," since it is only recently that we have learned, from careful observations on this subject in Europe, that eight is the mean number which may be seen in an hour in the field of vision of one individual (Quetelet, 'Corresp. Mathém.', Novem., 1837, p. 447); this number is, however, limited to five or six by that diligent observer, Olbers. (Schum., 'Jahrb.', 1838, s. 325.)

p 114 Sir Alexander Burnes likewise extols as a consequence of the purity of the atmosphere in Bokhara the enchanting and constantly-recurring spectacle of variously-colored shooting stars.

The connection of meteoric stones with the grander phenomenon of fire-balls — the former being known to be projected from the latter with such force as to penetrate from ten to fifteen feet into the earth — has been proved, among many other instances, in the falls of azzzuerolites at Barbotan, in the Department des Landes (24th July, 1790), at Siena (16th June, 1794), at Weston, in Connecticut, U. S. (14th December, 1807), and at Juvenas in the Department of Ardèche (14th June, 1821). Meteoric stones are in some instances thrown from dark clouds suddenly formed in a clear sky, and fall with a noise resembling thunder. Whole districts have thus occasionally been covered with thousands of fragmentary masses, of uniform character but unequal magnitudes, that p 115 have been hurled from one of these moving clouds. In less frequent cases, as in that which occurred on the 16th of September, 1843, at Kleinwenden, near Mühilhausen, a large aërolite fell with a thundering crash while the sky was clear and cloudless. The intimate affinity between fire-balls and shooting stars is further proved by the fact that fire-balls, from which meteoric stones have been thrown have occasionally been found, as at Angers, on the 9th of June, 1822, having a diameter scarcely equal to that of the small fire-works called Roman candles.

The formative power, and the nature of the physical and chemical processes involved in these phenomena are questions all equally shrouded in mystery, and we are as yet ignorant whether the particles composing the dense mass of meteoric stones are originally, as in comets, separated from one another when they become luminous to our sight, or whether in the case of smaller shooting stars, any compace substance actually falls, or, finally, whether a meteor is composed only of a smoke-like dust, containing iron and nickel; while we are wholly ignorant of what takes place within the dark cloud from which a noise like thunder is often heard for many minutes before the stones fall.*

[footnote] *On 'méteoric dust', see Arago, in the 'Annuaire' for 1832, p. 254. I haave very recently endeavored to show, in another work ('Asie Centrale', t. i., p. 408). how the Scythian saga of the sacred gold, which fell burning from heaven, and remained in the possession of the Golden Horde of the Paralatæ (Herod., iv., 5-7), probably originated in the vague recollection of the fall of an aërolite. The ancients had also some strange fictions (Dio Cassius, lxxv., 1259) or silver which had fallen from heaven, and with which it had been attempted, under the Emperor Severus, to cover bronze coins; metallic iron was however, known to exist in meteoric stones. (Plin., ii., 56.) The frequently-recurring expression 'lapidibus pluit' must not always be understood to refer to falls of aërolites. In Liv., xxv., 7, it probably refers to pumice ('rapilli') ejected from the volcano, Mount Albanus (Monte Cavo), which was not wholly extinguished at the time. (See Heyne, 'Opuscula Acad.', t. iii., p. 261; and my 'Relation Hist.', t. i., p. 394.) The contest of Hercules with the Ligyans, on the road from the Caucasus to the Hesperides, belongs to a different sphere of ideas, being an attempt to explain mythically the origin of the round quartz blocks in the Ligyan field of stones at the mouth of the Rhone, which Aristotle supposes to have been ejected from a fissure during an earthquake, and Posidonius to have been caused by the force of the waves of an inland piece of water. In the fragments that we still possess of the play of Æschylus, the 'Prometheus Delivered', every thing proceeds, however, in part of the narration, as in a fall of aërolites, for Jupiter draws together a cloud, and causes the "district around to be covered by a shower of round stones". Posidonius even ventured to deride the geognostic myth of the blocks and stones. The Lygian field of stones was, however, very naturally and well described by the ancients. The district is now known as 'La Crau.' (See Guerin, 'Mesures Barométriques dans les Alpes, et Météorologie d'Avignon', 1829, chap. xii., p. 115.)

p 116 We can ascertain by measurement the enormous, wonderful, and wholly planetary velocity of shooting stars, fire-valls and meteoric stones, and we can gain a knowledge of what is the general and uniform character of the phenomenon, but not of the genetically cosmical process and the results of the metamorphoses. If meteoric stones while revolving in space are already consolidated into dense masses,* less dense, however, p 117 than the mean density of the earth, they must be very small nuclei, which surrounded by inflammable vapor or gas, form the innermost part of fire-balls, from the height and apparent diameter of which we may, in the case of the largest, estimate that the actual diameter varies from 500 to about 2800 feet.

[footnote] *The specific weight of aërolites varies from 1.9 (Alais) to 4.3 (Tabor). Their general density may be set down as 3, water being 1. As to what has been said in the text of the actual diameters of fire-balls, we must remark, that the numbers have been taken from the few measurements that can be relied upon as correct. These give for the fire-ball of Weston, Connecticut (14th December, 1807), only 500; for that observed by Le Roi (10th July, 1771) about 1000 and for that estimated by Sir Charles Blagden (18th January, 1783) 2600 feet in diameter. Brandes ('Unterhaltungen' bd.i., s. 42) ascribes a diameter varying from 80 to 120 feet to shooting stars, and a luminous train extending from 12 to 16 miles. There are, however, ample optical causes for supposing that the apparent diameter of fire-balls and shooting stars has been very much overrated. The volume of the largest fire-ball yet observed can not be compared with that of Ceres, estimating generally so exact and admirable treatise, 'On the Connection of the Physical Sciences', 1835, p. 411.) With the view of elucidating what has been stated in the text regarding the large zërolite that fell into the bed of the River Narni, but has not again been found, I will give the passage made known by Pertz, from the 'Chronicon Benedicti, Monachi Sancti Andreæ in Mont Soracte', a MS. belonging to the tenth century, and preserved in the Chigi Library at Rome. The Barbarous Latin of that age has been left unchanged. "Anno 921, temporibus domini Johannis Decimi pape, in anno pontificatus illius 7 visa sunt signa. Nam juxta urben Romam lapides plurimi de cælo cadere visi sunt. In civilate quæ vocatur Narnia tam diri ac tetri, ut nihil aliud credatur, quam de infernalibus locis deducti essent. Nam ita ex illis lapidibus unus omnium maximum est, ut decidens in flumen Narnus, ad mensuram unius cubiti super aquas fluminus usque hodie videretur. Nam et ignitæita ut pene terra contingeret. AliAnno 921, temporibus domini Johannis Decimi pape, in anno pontificatus illius 7 visa sunt signa. Nam juxta urben Romam lapides plurimi de cælo cadere visi sunt. In civilate quæ vocatur Narnia tam diri ac tetri, ut nihil aliud credatur, quam de infernalibus locis deducti essent. Nam ita ex illis lapidibus unus omnium maximum est, ut decidens in flumen Narnus, ad mensuram unius cubiti super aquas fluminus usque hodie videretur. Nam et ignitæ ita ut pene terra contingeret. Ali cadentes," etc. (Pertz, 'Monum. Germ. Hist. Scriptores', t. iii., p. 715.) On the aërolites of gos Potamus, which fell, according to the Parian Chroniccle, in the 78 1 Olympiad, see Böckh, 'Corp. Inscr. Graec', t. ii., p. 302, 320, 340; also Aristot., 'Meteor.', i., 7 (Ideler's 'Comm.', t. i., p. 404-407); Stob., 'Eel. Phys.', i., 25, p. 508 (Heeren); Plut., 'Lys.', c. 12; Diog. Laert., ii., 10; and see, also, subsequent notes in this work. According to a Mongolisn tradition, a black fragment of a rock, forty feet in height, fell from heaven on a plain near the source of the Great Yellow River in Western China. (Abel Rémusat, in Lamétherie, 'Jour. de Phys.', 1819, Mai p. 264.)

The largest meteoric masses as yet known are those of Otumpa, in Chaco, and of Bahia, in Brazil, described by Rubi de Celis as being from 7 to 7 1/2 feet in length. The meteoric stone of gos Potamos, celebrated in antiquity, and even mentioned in the Chronicle of the Parian Marbles, which fell about the year in which Socrates was born, has been described as of the size of two mill-stones, and equal in weight to a full wagon load. Notwithstanding the failure that has attended the efforts of the African traveler, Brown, I do not wholly relinquish the hope that, even after the lapse of 2312 years, this Thracian meteoric mass, which it would be so difficult to destroy, may be found, since the region in which it fell is now bcome so easy of access to European travelers. The huge aërolite which in the beginning of the tenth century fell into the river at Narni, projected between three and four feet above the surface of the water, as we learn from a document lately discovered by Pertz. It must be remarked that these meteoric bodies, whether in ancient or modern times can only be regarded as the principal fragments of masses that have been broken up by the explosion either of a fire-ball of a dark cloud.

On considering the enormous velocity with which, as has been mathematically proved, meteoric stones reach the earth from the extremest confines of the atmosphere, and the lengthened course traversed by fire-balls through the denser strata of the air, it seems more than improbable that these metalliferous stony masses, containing perfectly-formed crystals of olivine, labradorite, and pyroxene, should in so short a period of time has been converted from a vaporous condition to a solid nucleus. Moreover, that which falls from meteoric masses, even where the internal composition is chemically different, exhibits almost always the peculiar character of a fragment, being of a prismatic or truncated pyramidal form, with broad, somewhat curved faces, and rounded angles. But whence comes this form, which was first recognized by Schreiber as characteristic of the 'severed' part of a rotating planetary body? Here, as in the sphere of organic life, all that appertains to the history of development remains hidden in obscurity. Meteoric masses become luminous and kindle at heights which p 118 must be regarded as almost devoid of air, of occupied by an atmosphere that does not even contain 1/100000th part of oxygen. The recent investigations of Biot on the important phenomenon of twilight* have considerably lowered the lines which had, perhaps with some degree of temerity, been usually termed the boundaries of the atmosphere; but processes of light may be evolved independently of the presence of oxygen, and Poisson conjectured that aëroliteswere ignited far beyond the range of our atmosphere. Numerical calculation and geometrical measurement are the only means by which as in the case of the larger bodies of our solar system, we are enabled to impart a firm and safe basis to our investigations of meteoric stones.

[footnote] *Biot, 'Traité d'Astronomie Physique' (3ème éd.), 1841, t. i., p. 149, 177, 238, 312. My lamented friend Poisson endeavored, in a singular manner, to solve the difficulty attending an assumption of the spontaneous ignition of meteoric stones at an elevation where the density of the atmosphere is almost null. These are his words: "It is difficult to attribute, as is uaually done, the incandescence of aërolites to friction against the molecules of the atmosphere at an elevation above the earth where the density of the air is almost null. May we not suppose that the electric fluid, in a neutral condition, forms a kind of atmosphere, extending far beyond the mass of our atmosphere, yet subject to terrestrial attraction, although physically imponderable, and consequently following our globe in its motion? According to this hypothesis, the bodies of which we have been speaking would, on entering this imponderable atmosphere, decompose the neutral fluid by their unequal action on the two electricities, and they would thus be heated, and in a state of incandescence, by becoming electrified." (Poisson, 'Rech. sur la Probabilité des Jugements', 1837, p. 6.)

Although Halley pronounced the great fire-ball of 1686, whose motion was opposite to that of the earth in its orbit,* to be a cosmical body, Chadni, in 1794, first recognized, with ready acuteness of mind, the connection between fire-balls and the stones projected from the atmosphere, and the motions of the former bodies in space.**

[footnote] *'Philos. Transact.', vol. xxix., p. 161-163.

[footnote] **The first edition of Chlandni's important treatise, 'Ueber den Ursprung der von Pallas gefundenen und anderen Eisenmassen' (On the Origin of the masses of Iron found by Pallas, and other similar masses), appeared two months prior to the shower of stones at Siena, and two years before Lichtenberg stated, in the 'Güttingen Taschenbuch', that "stones reach our atmosphere from the remoter regions of space.' Comp., also, Olbers's letter to Benzenberg, 18th Nov., 1837, in Benzenberg's 'Treatise on Shooting Stars', p. 186.

A brilliant confirmation of the cosmical origin of these phenomena has been afforded by Denison Olmsted, at New Haven, Connecticut, who has shown on the concurrent authority of all eye-witnesses, that during the celebrated fall of shooting stars on the night between the 12th p 119 and 13th of November, 1833, the fire-balls and shooting stars all emerged from one and the same quarter of the heavens, namely, in the vicinity of the star 'gamma' in the constellation Leo, and did not deviate from this point, although the star changed its apparent height and azimuth during the time of the observation. Such an independence of the Earth's rotation shows that the luminous body must have reached our atmosphere from 'without.' According to Encke's computation* of the whole p 120 number of observations made in the United States of North America, between the thirty-fifth and the forty-second degrees of latitude, it would appear that all these meteors came from the same point of space in the direction in which the Earth was moving at the time.

[footnote] *Encke, in Poggend., 'Annalen', bd. xxxiii. (1834), s. 213. Arago, in the 'Annuaire' for 1836, p. 291. Two letters which I wrote to Benzenberg, May 19 and October 22, 1837, on the conjectural precession of the nodes in the orbit of periodical falls of shooting stars. (Benzenberg's 'Sternsch.', s. 207 and 209.) Olbers subsequently adopted this opinion of the gradual retardation of the November phenomenon. ('Astron. Nachr.', 1838, No. 372, s. 180.) If I may venture to combine two of the falls of shooting stars mentioned by the Arabian writers with the epochs found by Boguslawski for the fourteenth century, I obtain the following more or less accordant elements of the movements of the nodes: In Oct., 902, on the night in which King Ibrahim ben Ahmed died, there fell a heavy shower of shooting stars, "like a fiery rain;" and this year was, therefore, called the year of stars. (Conde, 'Hist. de la Domin.' de los Arabes', p. 346.) On the 19th of Oct., 1202, the stars were in motion all night. "They fell like locusts." ('Comptes Rendus', 1837, t. i., p. 294; and Fræhn, in the 'Bull. de l'Académie de St. Pétersbourg', t. iii., p. 308.) On the 21st Oct., O.S., 1366, "'die sequente post festum XI. millia Virginum ab hora matutina usque ad horam primam visæ sunt quasi stellæ de cælo cadere continuo, et in tanta multitudine, quod nemo narrare suf ficit.'" This remarkable notice, of which we shall speak more fully in the subsequent part of this work, was found by the younger Von Boguslawski, in Benesse (de Horowic) de Weitmil or Weithmül, 'Chronicon Ecclesiæ Pragensis', p. 389. This chronicle may also be found in the second part of 'Scriptores rerum Bohemicarum', by Pelzel and Dobrowsky, 1784. (Schum., 'Astr. Nachr.', Dec., 1839.) On the night between the 9th and 10th of November, 1787, many falling stars were observed at Manheim, Southern Germany, by Hemmer (Kämtz, 'Meteor.', th. iii., s. 237.) After midnight, on the 12th of November, 1799, occurred the extraordinary fall of stars at Cumana, which Bonpland and myself have described, and which was observed over a great part of the earth. ('Relat. Hist.', t. i., p. 519-527.) Between the 12th and 13th of November, 1822, shooting stars, intermingled with fire-balls, were seen in large numbers by Kloden, at Potsdam. (Gilbert's 'Ann.', bd. lxxii., s. 291.) On the 13th of November, 1831, at 4 o'clock in the morning, a great shower of falling stars was seen by Captain Bérard, on the Spanish coast, near Carthagena del Levante. ('Annuaire', 1836, p. 297.) In the night between the 12th and 13th of November, 1833, occurred the phenomenon so admirably described by Professor Olmsted, in North America. In the night of the 13-14th of November, 1834, a similar fall of shooting stars was seen in North America, although the numbers were not quite so considerable. (Poggend., 'Annalen', bd. xxxiv., s. 129.) On the 13th of November, 1835, a barn was set on fire by the fall of a sporadic fire-ball, at Belley, in the Department de l'Ain. ('Annuaire', 1836, p. 296.) In the year 1838, the stream showed itself most decidedly on the night of the 13-14th of November. ('Astron. Nachr.', 1838, No. 372.)

On the recurrence of falls of shooting stars in North America, in the month of November of the years 1834 and 1837, and in the analogous falls observed at Bremen in 1838, a like general parallelism of the orbits, and the same direction of the meteors from the constellation Leo, were again noticed. It has been supposed that a greater parallelism was observable in the direction of periodic falls of shooting stars than in those of sporadic occurrence; and it has further been remarked, that in the periodically-recurring falls in the month of August, as, for instance, in the year 1839, the meteors came principally from one point between Perseus and Taurus, toward the latter of which constellations in the Earth was then moving. This peculiarity of the phenomenon, manifested in the retrograde direction of the orbits in November and August, should be thoroughly investigated by accurate observations, in order that it may either be fully confirmed or refuted.

The heights of shooting stars, that is to say, the heights of the points at which they begin and cease to be visible, vary exceedingly, fluctuating between 16 and 140 miles. This important result, and the enormous velocity of these problematical asteroids, were first ascertained by Benzenberg and Brandes, by simultaneous observations and determinations of parallax at the extremities of a base line of 49,020 feet in length.*

[footnote] *I am well aware that, among the 62 shooting stars simultaneously observed in Silesia, in 1823, at the suggestion of Professor Brandes some appeared to have an elevation of 183 to 240, or even 400 miles. (Brandes, 'Unterhaltungen für Freunde der Astronomie und Physik', heft i., s. 48. Instructive Narratives for the Lovers of Astronomy and Physics.) But Olbers considered that all determinations for elevations beyond 120 miles must be doubtful, owing to the smallness of the parallax.

The relative velocity of motion is from 18 to 36 miles in a second, and consequently equal to planetary velocity. This planetary velocity,* as well as the direction of the orbits p 121 of fire-balls and shooting stars, which has frequently been observed to be opposite to that of the Earth, may be considered as conclusive arguments against the hypothesis that aërolites derive their origin from the so-called active 'lunar volcanoes.'

[footnote] *The planetary velocity of translation, the movement in the orbit, is in Mercury 26.4, in Venus 19.2, and in the Earth 16.4 miles in a second.

Numerical views regarding a greater or lesser volcanic force on a small cosmical body, not surrounded by any atmosphere, must, from their nature, be wholly arbitrary. We may imagine the reaction of the interior of a planet on its crust ten or even a hundred times greater than that of our present terrestrial volcanoes; the direction of masses projected from a satellite revolving from west to east might appear retrogressive, owing to the Earth in its orbit subsequently reaching that point of space at which these bodies fall. If we examine the whole sphere of relations which I have touched upon in this work, in order to escape the charge of having made unproved assertions, we shall find that the hypothesis of the selenic origin of meteoric stones* depends upon a number of conditions p 122 whose accidental coincidence could alone convert a possible into an actual fact.

[footnote] *Chladni states that an Italian physicist, Paolo Maria Terzago, on the occasion of the fall of an aërolite at Milan in 1660, by which a Franciscan monk was killed, was the first who surmised that aërolites were of selenic origin. He says, in a memoir entitled 'Musæum Septalianum, Manfredi Septalæ, Patricii Mediolanensis, industrioso labore constructum' (Tortona, 1664, p. 44), "Labant philosophorum mentes sub horum lapidum ponderibus; ni dicire velimus, lunan terram alteram, sine mundum esse, ex cujus montibus divisa frustra in inferiorem nostrum hunc orben dela bantur." Without any previous knowledge of this conjecture, Olbers was led, in the year 1795 (after the celebrated fall at Siena on the 16th of June, 1794), into an investigation of the amount of the initial tangential force that would be requisite to bring to the Earth masses projected from the Moon. This ballistic problem occupied, during ten or twelve years, the attention of the geometricians Laplace, Biot, Brandes, and Poisson. The opinion which was then so prevalent, but which has since been abandoned, of the existence of active volcanoes in the Moon, where air and water are absent, led to a confusion in the minds of the generality of persons between mathematical possibilities and physical probabilities. Olbers, Brandes, and Chladni thought "that the velocity of 16 to 32 miles, with which fire-balls and shooting stars entered our atmosphere," furnished a refutation to the view of their selenic origin. According to Olbers, it would require to reach the Earth, setting aside the resistance of the air, an initial velocity of 8292 feet in the second; according to Laplace, 7862; to Biot, 8282; and to Poisson, 7595. Laplace states that this velocity is only five or six times as great as that of a cannon ball; but Olbers has shown "that, with such an initial velocity as 7500 or 8000 feet in a second, meteoric stones would arrive at the surface of our earth with a velocity of only 35,000 feet (or 1.53 German geographical mile). But the measured velocity of meteoric stones averages five such miles, or upward of 114,000 feet to a second; and, consequently, the original velocity of projection from the Moon must be almost 110,000 feet, and therefore fourteen times greater than Laplace asserted." (Olbers, in Schum, 'Jahrb.', 1837, p. 52-58; and in Gehler, 'Neues Physik.' 'Wörterbuche', bd. vi., abth.3, s. 2199-2136.) If we could assume volcanic forces to be still active on the Moon's surface, the absence of atmospheric resistance would certainly give to their projectile force an advantage over that of our terrestrial volcanoes; but even in respect to the measure of the latter force (the projectile force of our own volcanoes), we have no observations on which any reliance can be placed, and it has probably been exceedingly overrated. Dr. Peters, who accurately observed and measured the phenomena presented by Ætna, found that the greatest velocity of any of the stones projected from the crater was only 1250 feet to a second. Observations on the Peak of Teneriffe, in 1798, gave 3000 feet. Although Laplace, at the end of his work ('Expos. du Syst. du Monde', ed. de 1824, p. 399), cautiously observes, regarding aërolites, "that in all probability they come from the depths of space," yet we see from another passage (chap. vi., p. 233) 6that, being probably unacquainted with the extraordinary planetary velocity of meteoric stones, he inclines to the hypothesis of their lunar origin, always, however, assuming that the stones projjected from the Moon "become satellites of our Earth, describing around it more or less eccentric orbits, and thus not reaching its atmosphere until several or even many revolutions have been accomplished." As an Italian at Tortona had the fancy that aërolites came from the Moon, so some of the Greek philosophers thought they came from the Sun. This was the opinion of Diogenes Laertius (ii., 9) regarding the origin of the mass that fell at "gos Potamos (see note, p. 116). Pliny, whose labors in recording the opinions and statements of preceding writers are astonishing, repeats the theory, and derides it the more freely, because he, with earlier writers (Diog. Laert., 3 and 5, p. 99, Hübner), accuses Anaxagoras of having predicted the fall of aërolites from the Sun: "Celebrant Græci Anaxagoram Clazomenium Olympiadis septuagesimæ octavæ secundo anno prædixisse cælestium litterarum scientia quibus diebus saxum casurum esse e sole, idque factum interdia in Thraciæ parte ad gos flumen. Quod si quis prædictum credat, simul fateatur necesse est, majoris miraculi divinitatem Anaxagoræ fuisse, solvique rerum naturæ intellectum, et confundi omnia, si aut ipse Sol lapis esse aut unquam lapidem in eo fuisse credatur; decidere tamen crebro non erit dubium." The fall of a moderate-sized stone, which is preserved in the Gymnasium at Abydos, is also reported to have been foretold by Anaxagoras. The fall of aërolites in bright sunshine, and when the Moon's disk was invisible, probably led to the idea of sun-stones. Moreover, according to one of the physical dogmas of Anaxagoras, which brought on him the persecution of the theologians (even as they have attacked the geologists of our own times), the Sun was regarded as "a molten fiery mass" ([Greed words]). In accordance with these views of Anaxagoras, we find Euripides, in 'Phaëton', terming the Sun "a golden mass;" that is to say, a fire-colored, brightly-shining matter, but not leading to the inference that aërolites are golden sun-stones. (See note to page 115.) Compare Valckenaer, 'Diatribe in Eurip. perd. Dram. Reliquias', 1767, p. 30. Diog. Laert., ii., 40. Hence, among the Greek philosophers, we find four hypotheses regarding the origin of falling stars: a telluric origin from ascending exhalations; masses of stone raised by hurricane (see Aristot., 'Meteor., lib. i., cap. iv., 2-13, and cap. vii., 9); a solar origin; and, lastly, an origin in the regions of space, as heavenly bodies which had long remained invisible. Respecting this last opinion, which is that of Diogenes of Apollonia, and entirely accords with that of the present day, see pages 124 and 125. It is worthy of remark, that in Syria, as I have been assured by a learned Orientalist, now resident at Smyrna, Andrea de Nericat, who instructed me in Persian, there is a popular belief that aërolites chiefly fall on clear moonlight nights. The ancients, on the contrary, especially looked for their fall during lunar eclipses. (See Pliny, xxxvii., 10, p. 164. Solinus, c. 37. Salm., 'Exere.', p. 531; and the passages collected by Ukert, in his 'Geogr. der Griechen und Römer', th. ii., 1, s. 131, note 14.) On the improbability that meteoric masses are formed from metal-dissolving gases, which, according to Fusinieri, may exist in the highest strata of our atmosphere, and previously diffused through an almost boundless space, may suddenly assume a solid condition, and on the penetration and misceability of gases, see my ' Relat. Hist.', t. i., p. 525.

p 122 The view of the original existence of p 123 small planetary masses in space is simpler, and at the same time, more analogous with those entertained concerning the formation of other portions of the solar system.

It is very probable that a large number of these cosmical bodies traverse space undestroyed by the vicinity of our atmosphere, and revolve round the Sun without experiencing any alteration but a slight increase in the eccentricity of their orbits, occasioned by the attraction of the Earth's mass. We may, consequently, suppose the possibility of these bodied remaining invisible to us during many years and frequent revolutions. The supposed phenomenon of ascending shooting stars and fire-balls, which Chladni has unsuccessfully endeavored to explain on the hypothesis of the 'reflection' of strongly compressed air, appears at first sight as the consequence of some unknown tngential force propelling bodies from the earth; but Bessel has shown by theoretical deductions, confirmed by Feldt's carefully-conducted calculations, that, owing to the absence of any proofs of the simultaneous occurrence of the observed disappearances, the assumptiopn of an ascent of shooting stars was rendered wholly improbable, and inadmissible as a result of observation.*

[footnote] *Bessel, in Schum., 'Astr. Nachr.', 1839, No 389 und 381, s. 222 und 346. At the conclusion of the Memoir there is a comparison of the Sun's longitudes with the epochs of the November phenomenon, from the period of the first observations in Cumana in 1799,

The opinion advanced by Olbers that the explosion of shooting stars and ignited fire-balls not moving in straight lines may impel meteors upward in the manner of rockets, and influence the direction of their orbits, must be made the subject of future researches.

Shooting stars fall either seprately and in inconsiderable numbers, that is, sporadically, or in swarms of many thousands. p 124 The latter, which are compared by Arabian authors to swarms of locusts, are periodic in their occurrence, and move in streams, generally in a parallel direction. Among periodic falls, the most celebrated are that known as the November phenomenon, occurring from about the 12th to the 14th of November, and that of the festival of St. Lawrence (the 10th of August), whose "fiery tears" were noticed in former times in a church calendar of England, no less than in old traditionary legends, as a meteorological event of constant recurrence.*

[footnote] *Dr. Thomas Forster ('The Pocket Encyclopedia of Natural Phenomena' 1827, p. 17) states that a manuscript is preserved in the library of Christ's College, Cambridge,** written in the tenth century by a monk, and entitled 'Ephemerides Rerum Naturalium', in which the natural phenomena for each day of the year are inscribed as, for instance, the first flowering of plants, the arrival of birds, etc.; the 10th of August is distinguished by the word "meteorodes." It was this indication, and the tradition of the fiery tears of St. Lawrence, that chiefly induced Dr. Forster to undertake his extremely zealous investigation of the August phenomena. (Quetelet, 'Correspond. Mathém.', Série III., t. i., 1837, p. 433.)

[further footnote] **[No such manuscript is at present known to exist in the library of that college. For this information I am indebted to the inquiries of Mr. Cory, of Pembroke College, the learned editor of 'Hieroglyphics of Horapollo Nilous', Greek and English, 1840.] — Tr.

Notwithstanding the great quantity of shooting stars and fire-balls of the most various dimensions, which, according to Klöden, were seen to fall at Potsdam on the night between the 12th and 13th of November, 1822, and on the same night of the year in 1832 throughout the whole of Europe, from Portsmouth to Orenburg on the Ural River, and even in the southern hemisphere, as in the Isle of France, no attention was directed to the 'periodicity' of the phenomenon, and no idea seems to have been entertained of the connection existing between the fall of shooting stars and the recurrence of certain days, until the prodigious swarm of shooting stars which occurred in North America between the 12th and 13th of November, 1833, and was observed by Olmsted and Palmer. The stars fell on this occasion, like flakes of snow, and it was calculated that at least 240,000 had fallen during a period of nine hours. Palmer, of New Haven, Connecticut, was led, in consequence of this splendid phenomenon, to the recollection of the fall of meteoric stones in 1799, first described by Ellicot and myself,* and which, by p 125 a comparison of the facts I had adduced, showed that the phenomenon had been simultaneously seen in the New Continent, from the equator to New Herrnhut in Greenland (65 degrees 14' north latitude), and between 46 degrees and 82 degrees longitude.

[footnote] *Humb., 'Rel. Hist.', t. i., p. 519-527. Ellicot in the 'Transactions of the American Society', 1804, vol. vi., . 29. Arago makes the following observations in reference to the November phenomena: "We thus become more and more confirmed in the belief that there exists a zone composed of millions of small bodies, whose orbits cut the plane of the ecliptic at about the point which out Earth annually occupies between the 11th and 13th of November. It is a new planetary world beginning to be revealed to us." ('Annuaire', 1836, p. 296.)

The identity of the epochs was recognized with astonishment. The stream which had been seen from Jamaica to Boston (40 degrees 21' north latitude) to traverse the whole vault of heaven on the 12th and 13th of November, 1833, was again observed in the United States in 1834, on the night between the 13th and 14th of November, although on this latter occasion it showed itself with somewhat less intensity. In Europe the periodicity of the phenomenon has since been manifested with great regularity.

Another and a like regularly recurring phenomenon is that noticed in the month of August, the meteoric stream of St. Lawrence, appearing between the 9th and 14th of August. Muschenbrock,* as early as in the middle of the last century, drew attention to the frequency of meteors in the month of August' but their certain periodic return about the time of St. Lawrence's day was first shown by Quetelet, Olbers, and Benzenberg.

[footnote] *Compare Muschenbroek, 'Introd. ad Phil. Nat.', 1762, t. ii., p. 1061; Howard, 'On the Climate of London', vol. ii., p. 23, observations of the year 1806; seven years, therefore aftr the earliest observations of Brandes (Benzenberg, 'über Sternschnuppen', s. 240-244); the August observations of Thomas Forster, in Quetelet, op. cit., p. 438-453; those of Adolph Erman, Boguslawski, and Kreil, in Schum., 'Jahrb.', 1838, s. 317-330. Regarding the point of origin in Perseus, on the 10th of August, 1839, see the accurate measurements of Bessel and Erman (Schum., 'Astr. Nachr.', No. 385 und 428); but on the 10th of August, 1837, the path does not apper to have been retrograde; see Arago in 'Comptes Rendus', 1837, t. ii., p. 183.

We shall, no doubt, in time, discover other periodically appearing streams,* probably about the 22d to the p. 126 25th of April, between the 6th and 12th of December, and, to judge by the number of true falls of aërolites enumerated by Capocci, also between the 27th and 29th of November, of about the 17th of July.

[footnote] *On the 25th of April, 1095, "innumerable eyes in France saw stars falling from heaven as thickly as hail" ('ut grando, nisi lucerent, pro densitate putaretur'; Baldr., p. 88), and this occurrence was regarded by the Council of Clermont as indicative of the great movement in Christendom. (Wilken, 'Gesch. der Kreuzzüge', bd. i., s. 75.) On the 25th of April, 1800, a great fall of stars was observed in Virginia and Massachusetts; it was "a fire of rockets that lasted two hours." Arago was the first to call attention to the "trainée d'asteroïdes," as a recurring phenomenon. ('Annuaire', 1836, p. 297.) The falls of aërolites in the beginning of the month of December are also deserving of notice. In reference to their periodic recurrence as a meteoric stream, we may mention the early observation of Brandes on the night of the 6th and 7th of December, 1798 (when he counted 2000 falling stars), and very probably the enormous fall of aërolites that occurred at the Rio Assu, near the village of Macao, in the Brazils, on the 11th of December, 1836. (Brandes, 'Unterhalt. für Freunde der Physik', 1825, heft i., s. 65, and 'Comptes Rendus', t. v., p. 211.) Capocci, in the interval between 1809 and 1839, a space of thirty years, has discovered twelve authenticated cases of aërolites occurring between the 27th and 29th of November, besides others on the 13th of November, the 10th of August, and the 17th of July. ('Comptes Rendus', t. xi., p. 357.) It is singular that in the portion of the Earth's path corresponding with the months of January and February, and probably also with March, no 'periodic' streams of falling stars of aërolites have as yet been noticed; although when in the South Sea in the year 1803, I observed on the 15th of March a remarkably large number of falling stars, and they were seen to fall as in a swarm in the city of Quito, shortly before the terrible earthquake of Riobamba on the 4th of February, 1797. From the phenomena hitherto observed, the following epochs seem especially worthy of remark: 22d to the 25th of April. 17th of July (17th to the 26th of July?). (Quet., 'Corr.', 1837, p. 435.) 10th of August. 12th to the 14th of November. 27th to the 29th of November. 6th to the 12th of December. When we consider that the regions of space must be occupied by myriads of comets, we are led by analogy, notwithstanding the differences existing between isolated comets and rings filled with asteroids, to regard the frequency of these meteoric streams with less astonishment than the first consideration of the phenomenon would be likely to excite.

Although the phenomena hitherto observed appear to have been independent of the distance from the pole, the temperature of the air, and other climatic relations, there is, however, one perhaps accidentally coincident phenomenon which must not be wholly disregarded. The Northern Light, the Aurora Borealis, was unusually brilliant on the occurrence of the Borealis, was unusually brilliant on the occurrence of the splendid fall of meteors of the 12th and 13th November, 1833, described by Olmsted. It was also observed at Bremen in 1838, where the periodic meteoric fall was, however, less remarkable than at Richmond, near London. I have mentioned in another work the singular fact observed by Admiral Wrangel, and frequently confirmed to me by himself,* that when he p 127 was on the Siberian coast of the Polar Sea, he observed, during an Aurora Borealis, certain portions of the vault of heaven which were not illuminated, light up and continue luminous whenever a shooting star passed over them.

[footnote] *Ferd. v. Wrangle, 'Reise längs der Nordküste von Sibirien in den Jahren', 1820-1824, th. ii., s. 259. Regarding the recurrence of the denser swarm of the November stream after an interval of thirty-three years, see Olbers, in 'Jahrb.', 1837, s. 280. I was informed in Cumana that shortly before the fearful earthquake of 1766, and consequently thirty-three years (the same interval) before the great fall of stars on the 11th and 12th of November, 1799, a similar fiery manifestation had been observed in the heavens. But it was on the 21st of October, 1766, and not in the beginning of November, that the earthquake occurred. Possibly some traveler in Quito may yet be able to ascertain the day on which the volcano of Cayambe, which is situated there, was for the space of an hour enveloped in falling stars, so that the inhabitants endeavored to appease heaven by religious processions. ('Relat. Hist.', t. i., chap. iv., p 307; chap. x., p. 520 and 527.)

The different meteoric streams, each of which is composed of myriads of small cosmical bodies, probably intersect our Earth's orbit in the same manner as Biela's comet. According to this hypothesis, we may represent to ourselves these asteroid-meteors as composing a closed ring or zone, within which they all pursue one common orbit. The s aller planets between Mars and Jupiter present us if we except Pallas with an analogous relation in their constantly intersecting orbits. As yet, however, we have no certain knowledge as to whether changes in the periods at which the stream becomes visible, or the 'retardations' of the phenomena of which I have already spoken, indicate a regular precession of oscillation of the nodes — that is to say, of the points of intersection of the Earth's orbit and of that of the ring; or whether this ring or zone attains so considerable a degree of breadth from the irregular grouping and distances apart of the small bodies, that it requires several days for the Earth to traverse it. The system of Saturn's satellites shows us likewise a group of immense width, composed of most intimately-connected cosmical bodies. In this system, the orbit of the outermost (the seventh) satellite has such a vast diameter, that the Earth, in her revolution round the Sun, requires three days to traverse an extent of space equal to this diameter. If, therefore, in one of these rings, which we regard as the orbit of a periodical stream, the asteroids should be so irregularly distributed as to consist of but few groups sufficiently dense to give rise to these phenomena, we may easily understand why we so seldom witness such glorious spectacles as those exhibited in the November months of 1799 and 1833. The acute mind of Olbers led him almost to predict that the next appearance of the phenomenon of shooting stars and fire-balls intermixed, falling like flakes of snow, would not recur until between the 12th and 14th of November, 1867.

p 128 The stream of the November asteroids has occasionally only been visible in a small section of the Earth. Thus, for instance, a very splendid 'meteoric shower' was seen in England in the year 1837, while a most attentive and skillful observer at Braunsberg, in Prussia only saw on the same night, which was there uninterruptedly clear, a few sporadic shooting stars fall between seven o'clock in the evening and sunrise the next morning. Bessel* concluded from this "that a dense group of the bodies composing the great ring may have reached that part of the Earth in which England is situated, while the more eastern districts of the Earth might be passing at the time through a part of the meteoric ring proportionally less densely studded with bodies."

[footnote] *From a letter to myself, dated Jan. 24th, 1838. The enormous swarm of falling stars in November, 1799, was almost exclusively seen in America, where it was witnessed from New Herrnhut in Greenland to the equator. The swarms of 1831 and 1832 were visible only in Europe, and those of 1833 and 1834 only in the United States of North America.

If the hypothesis of a regular progression or oscillation of the nodes should acquire greater weight, special interest will be attached to the investigation of older observations. The Chinese annals, in which great falls of shooting stars, as well as the phenomena of comets, are recorded, go back beyond the age of Tyrtæs, or the second Messenian war. They give a description of two streams in the month of March, one of which is 687 years anterior to the Christian era. Edward Biot has observed that among the fifty-two phenomena which he has collected from the Chinese annals, those that were of most frequent recurrence are recorded at periods nearly corresponding with the 20th and 22d of July, O.S., and might consequently be identical with the stream of St. Lawrence's day, taking into account that it has advanced since the epochs* indicated.

[footnote] *Lettre de M. Edouard Biot à M. Quetelet, sur les anciennes apparitions d'Etoiles Filantes en Chine, in the 'Bull. de l'Académie de Bruxelles', 1843, t. x., No. 7, p. 8. On the notice from the 'Chronicon Ecclesiæ Pragensis', see the younger Boguslawski, in Poggend., 'Annalen', bd. xlviii., s. 612.

If the fall of shooting stars of the 21st of October, 1366, O.S. (a notice of which was found by the younger Von Boguslawski, in Benessius de Horowic's 'Chronicon Ecclesiæ Pragensis'), be identical with our November phenomenon, although the occurrence in the fourteenth century was seen in broad daylight, we find by the precession in 477 years that this system of meteors, or, rather, its common center of gravity, must describe p 129 a retrograde orbit round the Sun. It also follows, from the views thus developed, that the non-appearance, during certain years, in any portion of the Earth, of the two streams hitherto observed in November and about the time of St. Lawrence's day, must be ascribed either to an interruption in the meteoric ring, that is to say, to intervals occurring between the asteroid groups, or, according to Poisson to the action of the larger planets* on the form and position of this annulus.

[footnote] *"It appears that an apparently inexhaustible number of bodies, too small to be observed, are moving in the regions of space, either around the Sun or the planets, or perhaps even around their satellites. It is supposed that when these bodies come in contact with our atmosphere, the difference between their velocity and that of our planet is so great, that the friction which they experience from their contact with the air heats them to incandescence, and sometimes causes their explosion. If the group of falling stars form an annulus around the Sun, its velocity of circulation may be very different from that of our Earth; and the displacements it may experience in space, in consequence of the actions of the various planets, may render the phenomenon of its intersecting the planes of the ecliptic possible at some epochs, and altogether impossible at others." — Poisson, 'Recherches sur la Probabilité des Jugements', p. 306, 307.

The solid masses which are observed by night to fall to the earth from fire-balls, and by day generally when the sky is clear, from a cark small cloud, are accompanied by much candescence. They undeniably exhibit a great degree of general identity with respect to their external form, the character of their crust, and the chemical composition of their principal constituents. These characteristics of identity have been observed at all the different epochs and in the most various parts of the earth in which these meteoric stones have been found. This striking and early-observed analogy of physiognomy in the denser meteoric masses is, however, met by many exceptions regarding individual points. What differences, for instance, do we not find between the malleable masses of for instance, do we not find between the malleable masses of iron of Hradeschina in the district of Agram, those from the shores of the Sisim in the government of Jeniseisk, rendered so celebrated by Pallas, or those which I brought from Mexico,* all of which contain 96 per cent. of iron, from the aërolites of Siena, in which the iron scarcely amounts to 2 per cent., or the earthy aërolite of Alais (in the Department du Gard), which broke up in water, or, lastly, from those of Jonzac and Javenas, which contained no metallic iron, but presented a p 130 mixture of oryctognostically distinct crystalline compoonents!

[footnote] *Humboldt, 'Essai Politique sur la Nouv. Espagne' (2de édit.), t. iii. p. 310.

These differences have led mineralogists to separate these cosmical masses into two classes, namely, those containing nickelliferous meteoric iron, and those consisting of fine or coarsely-granular meteoric dust. The crust or rind of aërolites is peculiarly characteristic of these bodies, being only a few tenths of a line in thickness, often glossy and pitch-like, and occasionally veined.*

[footnote] *The peculiar color of their crust was observed even as early as in the time of Pliny (ii., 56 and 58): "colore adusto." The phrase "lateribus pluisse" seems also to refer to the burned outer surface of aërolites.

There is only one instance on record, as far as I am aware (the aërolite of Chantonnay, in La Vendée), in which the rind was absent, and this meteor, like that of Juvenas, presented likewise the peculiarity of having pores and vesicular cavities. In all other cases the black crust is divided from the inner light-gray mass by as sharply-defined a line of separation as is the black leaden-colored investment of the white granit blocks* which I brought from the cataracts of the Orinoco, and which are also associated with many other cataracts, as, for instance, those of the Nile and of the Congo River.

[footnote] * Humb., 'Rel. Hist.', t. ii., chap xx., p. 299-302.

The greatest heat employed in our porcelain ovens would be insufficient to produce any thing similar to the crust of meteoric stones, whose interior remains wholly unchanged. Here and there, facts have been observed which would seem to indicate a fusion together of the meteoric fragments; but, in general, the character of the aggregate mass, the absence of compression by the fall, and the inconsiderable degree of heat possessed by these bodies when they reach the earth, are all opposed to the hypothesis of the interior being in a state of fusion during their short passage from the boundary of the atmosphere to our Earth.

The chemical elements of which these meteoric masses consist, and on which Berzelius has thrown so much light, are the same as those distributed throughout the earth's crust, and are fifteen in number, namely, iron, nickel, cobalt, manganese, chromium, copper, arsenic, zinc, potash, soda, sulphur, phosphorus, and carbon, constituting altogether nearly one third of all the known simple bodies. Notwithstanding this similarity with the primary elements into which inorganic bodies are chemically reducible, the aspect of aërolites, owing to the mode in which their constituent parts are compounded, presents, generally, some features foreign to our telluric rocks and minerals. The pure native iron, which is almost always p 131 found incorporated with aërolites, imparts to them a peculiar, but not consequently, a 'selenic' character; for in other regions of space, and in other cosmical bodies besides our Moon, water may be wholly absent, and processes of oxydation of rare occurence.

Cosmical gelatinous vesicles, similar to the organic 'nostoc' (masses which have been supposed since the Middle Ages to be connected with shooting stars), and those pyrites of Sterlitamak, west of the Uralian Mountains, which are said to have constituted the interior of hailstones,* must both be classed among the mythical fables of meteorology.

[footnote] *Gustav Rose, 'Reise nach dem Ural', bd. II., s. 202.

Some few aërolites, as those composed of a finely granular tissue of olivine, augite, and labradorite blended together* (as the meteoric stone found at Juvenas, in the Department de l'Ardèche, which resembled dolorite), are the only ones, as Gustav Rose has remarked, which have a more familiar aspect.

[footnote] *Gustav Rose, in Poggend., 'Ann.', 1825, bd. iv., x. 173-192. Rammelsberg, 'Erstes Suppl. zum chem. Handwörterbuche der Mineralogie', 1843, s. 102. "It is," says the clear-minded observer Olbers, "a remarkable but hitherto unregarded fact, that while shells are found in secondary and tertiary formations, no 'fossil meteoric stones' have as yet been discovered. May we conclude from this circumstance that previous to the present and last modification of the earth's surface no meteoric stones fell on it, although at the present time it appears probable, from the researches of Schreibers, that 700 fall annually?" (Olbers, in Schum., 'Jahrb.', 1838, s. 329.) Problematical nickelliferous masses of native iron have been found in Northern Asia (at the gold-washing establishment at Petropawlowsk, eighty miles southeast of Kusnezk), imbedded thirty-one feet in the ground, and more recently in the Western Carpathians (the mountain chain of Magura, at Szlanicz), both of which are remarkably like meteoric stones. Compart Erman, 'Archiv für wissenschaftliche Kunde von Russland', bd. i., s. 315, and Haidinger, 'Bericht über Szlaniczer Schürfe in Ungarn.'

These bodiescontain, for instance, crystalline substances, perfectly similar to those of our earth's crust; and in the Siberian mass of meteoric iron investigated by Pallas, the olivine only differs from common olivine by the absence of nickel, which is replaced by the oxyd of tin.*

[footnote] *Berzelius, 'Jahresber.', bd. xv., s. 217 und 231. Rammelsberg, 'Handwörterb., abth. ii., s. 25-28.

As meteoric olivine, like our basalt, contains from 47 to 49 per cent. of magnesia, constituting, according to Berzelius, almost the half of the earthy components of meteoric stones, we can not be surprised at the great quantity of silicate of magnesia found in these cosmical bodies. If the zërolite of Juvenas contain separable crystals of augite and labradorite, the numerical relation of the constituents p 132 render it at least probable that the meteoric masses of Chateau-Renard may be a compound of diorite, consisting of hornblende and albite, and those of Blansko and Chantonnay compounds of hornblende and labradorite. The proofs of the telluric and atmospheric origin of aUerolites, which it is attempted to base upon the oryctognostic analogies presented by these bodies, do not appear to me to possess any great weight.

Recalling to mind the remarkable interview between Newton and Conduit at Kensington,* I would ask why the elementary substances that compose one group of cosmical bodies, or one planetary system, may not, in a great measure, be identical?

[footnote] * "Sir Isaac Newton said he took all the planets to be composed of the same matter with the Earth, viz., earth, water, and stone, but variously connected." — Turner, 'Collections for the History of Grantham, containing authentic Memoirs of Sir Isaac Newton', p. 172.

Why should we not adopt this view, since we may conjecture that these planetary bodies, like all the larger or smaller agglomerated masses revolving round the sun, have been thrown off from the once far more expanded solar atmosphere, and been formed from vaporous rintgs describing their orbits round the central body? We are not, it appears to me, more justified in applying the term telluric to the nickel and iron, the olivine and pyroxene (augite), found in meteoric stones, than in indicating the German plants which I found beyond the Obi as European species of the flora of Northern Asia. If the elementary substances composing a group of cosmical bodies of different magnitudes be identical, why should they not likewise, in obeying the laws of mutual attraction, blend together under definite relations of mixture, composing the white glittring snow and ice in the polar zones of the planet Mars, or constituting in the smaller cosmical masses mineral bodies inclosing crystals of olivine, augite, and labradorite? Even in the domain of pure conjecture we should not suffer ourselves to be led away by unphilosophical and arbitrary views devoid of the support of inductive reasoning.

Remarkable obscurations of the sun's disk, during which the stars have been seen at mid-day (as, for instance, in the obscuration of 1547, which continued for three days, and occurred about the time of the eventful battle of Mühlberg), can not be explained as arising from volcanic ashes or mists, and were regarded by Kepler as owing either to a 'materia cometica', or to a black cloud formed by the sooty exhalations of the solar body. The shorter obscurations of 1090 and 1203, which continued, the one only three, and the other six p 133 hours, were supposed by Chladni and Schnurrer to be occasioned by the passage of meteoric masses before the sun's disk. Since the period that streams of meteoric shooting stars were first considered with reference to the direction of their orbit as a closed ring, the epochs of these mysterious celestial phenomena have been observed to present a remarkable connection with the regular recurrence of swarms of shooting stars Adolph Erman has evinced great acuteness of mind in his accurate investigation of the facts hitherto observed on this subject, and his researches have enabled him to discover the connection of the sun's conjunction with the August asteroids on the 7th of February, and with the November asteroids on the 12th of May, the latter period corresponding with the days of St. Mamert (May 11th), St. Pancras (May 12th), and St. Servatius (May 13th), which according to popular belief, were accounted "cold days."*

[footnote] Adolph Erman, in Poggend., 'Annalen', 1839, bd. xlviii., s. 582-601. Biot had previously thrown doubt regarding the probability of the November stream reappearing in the beginning of May ('Comptes Rendus', 1836, t. ii., p. 670). Mädler has examined the mean depression of temperature on the three ill-named days of May by Berlin observations for eighty-six years ('Verhandl. des Vereins zur Bedförd, des Gartenbaues', 1834, s. 377), and found a retrogression of temperature amounting to 2.2 degrees Fahr. from the 11th to the 13th of May, a period at which nearly the most rapid advance of heat takes place. It is much to be desired that this phenomenon of depressed temperature, which some have felt inclined to attribute to the melting of the ice in the northeast of Europe, should be also investigated in very remote spots, as in America, or in the southern hemisphere. (Comp. 'Bull. de l'Acad. Imp. de St. Pétersbourg', 1843, t. i., No. 4.)

The Greek natural philosophers, who were but little disposed to pursue observations, but evinced inexhaustible fergility of imagination in giving the most various interpretation of half-perceived facts, have, however, left some hypotheses regarding shooting stars and meteoric stones which strikingly accord with the views now almost universally admitted of the cosmical process of these phenomena. "Falling stars," says Plutarch, in his life of Lysander,* are, according to the opinion of some physicists, not eruptions of the ethereal fire extinguished in the air immediately after its ignition, nor yet an inflammatory combustion of the air, which is dissolved in large quantities in the upper regions of space, but these meteors are rather a fall of celestial bodies, which, in consequence of a certain intermission in the rotatory force, and by the impulse of some irregular movements, have been hurled down not only to the inhabited portions of the Earth, but also beyond it into the great ocean, where we can not find them."

[footnote] *Plut., 'Vitæ par, in Lysandro', cap. 22. The statement of Damachos (Daïmachos), that for seventy days continuously there was a fiery cloud seen in the sky, emitting sparks like falling stars, and which then, sinking nearer to the earth, let fall the stone of Ægos Potamos, "which, however, was only a small part of it," is extremely improbable, since the direction and velocity of the fire-cloud would in that case of necessity have to remain for so many days the same as those of the earth; and this, in the fire-ball of the 19th of July, 1686, described by Halley ('Trans.', vol. xxix., p. 163), lasted only a few minutes. It is not altogether certain whether Daïmachos, the writer, [Greek words], was the same person as Daïmachos of Platæa, who was sent by Selencus to India to the son of Androcottos, and who ws charged by Strabo with being "a speaker of lies" (p. 70, Casaub.). From another passage of Plutarch ('Compar. Solonis c. Cop.', cap. 5) we should almost believe that he was. At all events, we have here only the evidence of a very late author, who wrote a century and a half after the fall of aërolites occurred in Thrace, and whose authenticity is also doubted by Plutarch.

Diogenes of Apollonia* expresses himself still more explicitly.

[footnote] *Stob., ed. Heeren, i., 25, p. 508; Plut., 'de plac. Philos.', ii., 13.

According to his views, "Stars that are 'invisible', and, consequently, have no name, move in space together with those that are visible. These invisible stars frequently fall burning at Ægos Potamos." The Apollonian, who held all other stellar bodies, when luminous, to be of a pumice-like nature, probably grounded his opinions regarding shooting stars and meteoric masses on the doctrine of Anaxagoras the Clazomenian, who regarded all the bodies in the universe "as fragments of rocks, which the fiery ether, in the force of its gyratory motion, had torn from the Earth and converted into stars." In the Ionian school, therefore, according to the testimony transmitted to us in the views of Diogenes of Apollonia, aërolites and stars were ranged in one and the same class; both, when considered with reference to their primary origin, being equally telluric, this being understood only so far as the Earth was then regarded as a central body,* p 135 forming all things around it in the same manner was we, according to our present views, suppose the planets of our system to have originated in the expanded atmosphere of another central body, the Sun.

[footnote] *The remarkable passage in Plut., 'de plac. Philos.', ii., 13, runs thus: "Anaxagoras teaches that the surrounding ether is a fiety substance, which, by the power of its rotation, tears rocks from the earth, inflames them, and converts them into stars." Applying an ancient fable to illustrate a physical dogma, the Clazomenian appears to have ascribed the fall of the Nemæan Lion to the Peloponnesus from the Moon to such a rotatory or centrifugal force. (Ælian., xii., 7; Plut., 'de Facie in Orge Lunæ' c. 24; Schol. ex Cod. Paris., in 'Apoll. Argon.', lib. i., p. 498, ed. Schaef., t. ii., p. 40; Meineke, 'Annal. Alex.', 1843, p. 85.) Here, instead of stones from the Moon, we have an animal from the Moon! According to an acute remark of Böckh, the ancient mythology of the Nemæan lunar lion has an astronomical origin, and is symbolically connected in chronology with the cycle of intercalation of the lunar year, with the moon-worship at Nemæa, and the games by which it was accompanied.

These views must not, therefore, be confounded with what is commonly termed the telluric or atmospheric origin of meteoric stones, nor yet with the singular opinion of Aristotle, which supposed the enormous mass of Ægos Potamos to have been raised by a hurricane. That rrogant spirit of incredulity, which rejects facts without attempting to investigate them, is in some cases almost more injurious than an unquestioning credulity. Both are alike detrimental to the force of investigation. Notwithstanding that for more than two thousand years the annals of different nations had recorded falls of meteoric stones, many of which had been attested beyond all doubt by the evidence of irreproachable eye-witnesses — notwithstanding the important part enacted by the Bætylia in the meteor-worship of the ancients — notwithstanding the fact of the companions of Cortez having see an aërolite at Cholula which had fallen on the neighboring pyramid — notwithstanding that califs and Mongolian chiefs had caused swords to be forged from recently-fallen meteoric stones — nay, notwithstanding that several persons had been struck dead by stones falling from heaven, as for instance, a monk at Crema on the 4th of September, 1511, another monk at Milan in 1650, and two Swedish sailors on board ship in 1674, yet this great cosmical phenomenon remained almost wholly unheeded, and its intimate connection drawn to the subject by Chladni, who had already gained immortal renown by his discovery of the sound-figures. He who is penetrated with a sense of this mysterious connection, and whose mind is open to deep impressions of nature, will feel himself moved by the deepest and most solemn emotion at the sight of every star that shoots across the vault of heaven, no less than at the glorious spectacle of meteoric swarms in the November phenomenon or on St. Lawrence's day. Here motion is suddenly revealed in the midst of nocturnal rest. The still radiance of the vault of heaven is for a moment animated with life and movement. In the mild radiance left on the track of the shooting star, imagination pictures the lengthened path of the meteor through the vault of heaven, p 136 while, every where around, the luminous asteroids proclaim the existence of one common material universe.

If we compare the volume of the innermost of Saturn's satellites, or that of Ceres, with the immense volume of the Sun, all relations of magnitude vanish from our minds. The extinction of suddenly resplendent stars in Cassiopeia, Cygnus, and Serpentarius have already led to the assumption of other and non-luminous cosmical bodies. We now know that the meteoric asteroids, spherically agglomerated into small masses, revolve round the Sun, intersect, like comets, the orbits of the luminous larger planets, and become ignited either in the vicinity of our atmosphere or in its upper strata.

The only media by which we are brought in connection with other planetary bodies, and with all portions of the universe beyond our atmosphere, are light and heat (the latter of which can scarcely be separated from the former),* and those mysterious powers of attraction exercised by remote masses, according to the quantity of their constituents, upon our globe, the ocean, and the strata of our atmosphere.

[footnote' *The following remarkable passage on the radiation of heat from the fixed stars, and on their low combustion and vitality — one of Kepler's many aspirations — occurs in the 'Paralipom. in Vitell. Astron. parsOpticqa', 1604, Propos. xxxii., p. 25: "Luciis proprium est calor, sydera omnia calefaciunt. De syderum luce claritatis ratio testatur, calorem universorum in minori esse proportione ad calorem unius solis, quam ut ab homine, cujus est certa caloris mensura, utrque simul percipi et judicari possit. De cincindularum lucula tenuissima negare non potes, quin cum calore sit. Vivunt enim et moventur, hoc auten non sine calefactione perficitur. Sic neque putrescentium lignorum lux sui calore destituitur; nam ipsa puetredo quidam lentus ignis est. Inest et stirpibus suus calor." (Compare Kepler, 'Epit. Astron. Copernicanæ', 1618, t. i., lib. i., p. 35.)

Another and different kind of cosmical, or, rather, material mode of contact is, however, opened to us, if we admit falling stars and meteoric stones to be planetary asteroids. They not only act upon us merely from a distance by the excitement of luminous or calorific vibrations, or in obedience to the laws of mutual attraction, but they acquire an actual material existence for us, reaching our atmosphere from the remoter regions of universal space, and remaining on the earth itself. Meteoric stones are the only means by which we can be brought in possible contact with that which is foreign to our own planet. Accustomed to gain our knowledge of what is not telluric solely through measurement, calculations, and the deductions of reason, we experience a sentiment of astonishment at finding that we may examine, weigh, and analyze bodies that appertain p 137 to the outer world. This awakens, by the power of the imagination, a meditative, spiritual train of thought, where the untutored mind perceives only scintillations of light in the firmament, and sees in the blackened stone that falls from the exploded cloud nothing beyond the rough product of a powerful natural force.

Although the asteroid-swarms, on which we have been led, from special predilection, to dwell somewhat at length, approximate to a certain degree, in their inconsiderable mass and the diversity of their orbits, to comets, they present this essential difference from the latter bodies, that our knowledge of their existence is almost entirely limited to the moment of their destruction, that is, to the period when, drawn within the sphere of the Earth's attraction they become luminous and ignite.

In order to complete our view of all that we have learned to consider as appertaining to our solar system, which now, since the discovery of the small planets, of the interior comets of short revolutions, and of the meteoric asteroids, is so rich and complicated in its form, it remains for us to speak of the ring of Zodiacal light, to which we have already alluded. Those who have lived for many years in the zone of palms must retain a pleasing impression of the mild radiance with which the zodiacal light, shooting pyramidally upward, illumines a part of the uniform length of tropical nights. I have seen it shine with an intensity of light equal to the milky way in Sagittarius, and that not only in the rare and dry atmosphere of the summits of the Andes, at an elevation of from thirteen to fifteen thousand feet, but even on the boundless grassy plains, the Illanos of Venezuela, and on the sea-shore, beneath the ever-clear sky of Cumana. This phenomenon was often rendered especially beautiful by the passage of light, fleecy clouds, which stood out in picturesque and bold relief from the luminous back-ground. A notice of this aërial spectacle is contained in a passage in my journal, while I was on the voyage from Lima to the western coasts of Mexico: "For three or four nights (between 10ºdegrees and 14ºdegrees north latitude) the zodiacal light has appeared in greater splendor than I have ever observed it. The transparency of the atmosphere must be remarkably great in this part of the Southern Ocean, to judge by the radiance of the stars and nebulous spots. From the 14th to the 19th of March a regular interval of three quarters of an hour occurred between the disappearance of the sun's disk in the ocean and the first manifestation of the zodiacal p 138 light, although the night was already perfectly dark. an hour after sunset it was seen in great briliancy between Aldebaran and the Pleiades; and on the 18th of March it attained an altitude of 39ºdegrees5'minutes. Narrow elongated clouds are scattered over the beautiful deep azure of the distant horizon, flitting past the zodiacal light as before a golden curtain. Above these, other clouds are from time to time reflecting the most brightly variegated colors. It seems a second sunset. On this side of the vault of heaven the lightness of the night appears to increase almost as much as at the first quarter of the moon. Toward 10 o'clock the zodiacal light generally becomes very faint in this part of the Southern Ocean, and at midnight I have scarcely been able to trace a vestige of it. On the 16th of March, when most strongly luminous a faint reflection was visible in the east." In our gloomy so-called "temperate" northern zone, the zodiacal light is only distinctly visible in the beginning of Spring, after the evening twilight, in the western part of the sky, and at the close of Autumn, before the dawn of day, above the eastern horizon.

It is difficult to understand how so striking a natural phenomenon should have failed to attract the attention of physicists and astronomers until the middle of the seventeenth century, or how it could have escaped the observation of the Atabian natural philosophers in ancient Bactria, on the euphrates, and in the south of Spain. Almost equal surprise is excited by the tardiness of observation of the nebulous spots in Andromeda and Orion, first described by Simon Marius and Huygens. The earliest explicit descriptions of the zodiacal light occurs in Childrey's 'Britannia Baconica',* in the year 1661. p 139

[footnote] *"There is another thing which I recommend to the observation of mathematical men, which is that in February, and for a little before and a little after that month (as I have observed several years together), about six in the evening, when the twilight hath almost deserted the horizon, you shall see a plainly discernible way of the twilight striking up toward the Pleiades, and seeming almost to touch them. It is so observed any clear night, but it is best illac nocte. There is no such way to be observed at any other time of the year (that I can perceive), nor any other way at that time to be perceived darting up elsewhere; and I believe it hath been, and will be constantly visible at that time of the year; but what the cause of it in nature should be, I can not yet imagine, but leave it to future inquiry." (Childrey, 'Britannia Baconica', 1661, p. 183.) This is the first view and a simple description of the phenomenon. (Cassini, 'Découverte de la Lumi dfd éleste qui paroît dans le Zodiaque', in the 'Mém. de l'Acad.', t. viii., 1730, p 276. Mairan, 'TraitéPhys de l'Aurore Boréale', 1754, 0. 16.) In this remarkable work by Childrey there are to be found (p. 91) very clear accounts of the epochs of maxima and minima diurnal and annual temperatures, and of the retardation of the extremes of the effects in meteorological processes. It is, however, to be regretted that our Baconian-philosophy-loving author, who was Lord Henry Somerset's chaplain, fell into the same error as Bernardin de St. Pierre, and regarded the Earth as elongated at the poles (see p. 148). At the first he believes that the Earth was spherical, but supposes that the uninterrupted and increasing addition of layers of ice at both poles has changed its figure; and that as the ice is formed from water, the quantity of that liquid is every where diminishing.

The first observation of the phenomenon may have been made two or three years prior to this period; but, notwithstanding, the merit of having (in the spring of 1683) been the first to investigate the phenomenon in all its relations in space is incontestably due to Dominicus Cassini. The light which he saw at Bologna in 1668, and which was observed at the same time in Persia by the celebrated traveler Chardin (the court astrologers of Ispahan called this light, which had never before been observed, 'nyzek', a small lance), was not the zodiacal light, as has often been asserted,* but the p 140 enormous tail of a comet, whose head was concealed in the vapory mist of the horizon, and which, from its length and appearance, presented much similarity to the great comet of 1843.

[footnote] *Dominicus Cassini ('Mém. de l'Acad.', t. viii., 1730, p. 188), and Mairan ('Aurore Bor.', p. 16), have even maintained that the phenomenon observed in Persia in 1668 was the zodiacal light. Delambre ('Hist. de l'Astron. Moderne', t. ii., p. 742), in very decided trms ascribes the discovery of this light to the celebrated traveler Chardin; but in the 'Couronnement de Soliman', and in several passages of the narrative of his travels (éd. de Langlès. t. iv., p. 326; t. x., p. 97), he only applies the term niazouk (nyzek), or "petite lance," to "the great and famous comet which appeared over nearly the whole world in 1668, and whose head was so hidden in the wewst that it could not be perceived in the horizon of Ispahan" ('Atlas du Voyage de Chardin', Tab. iv.; from the observations at Schiraz). The head or nucleus of the comet was, however, visible in the Brazils and in India (Pingré, 'Cométogr.', t. ii., p. 22). Regarding the conjectured identity of the last great comet of March, 1843, with this, which Cassini mistook for the zodiacal light, see Schum., 'Astr. Nachr.', 1843, No. 476 and 480. In Persian, the term "nizehi âteschîn"(fiery spears or lances) is also applied to the rays of the rising or setting sun, in the same way as "nayâzik," according to Freytag's Arabic Lexicon, signifies "stellæ cadentes." The comparison of comets to lances and swords was, however, in the Middle Ages, very common in all languages. The great comet of 1500, which was visible from April to June, was always termed by the Italian writers of that time 'il Signor Astone' (see my 'Examen Critique de l'Hist. de la Géographie', t. v., p. 80). All the hypotheses that have been advanced to show that Descartes (Cassini, p. 230; Mairan, p. 16), and even Kepler (Delambre, t. i., p. 601), were acquainted with the zodiacal light, appear to me altogether untenable. Descartes ('Principes', iii., art. 136, 137) is very obscure in his remarks on comets, observing that their tails are formed "by oblique rays, which, falling on different parts of the planetary orbs, strike the eye laterally by extraordinary refraction," and that they might be seen morning and evening, "like a long beam," when the Sun is between the comet and the Earth. This passage no more refers to the zodiacal light than those in which Kepler ('Epit. Astron. Copernicanæ', t. i., p. 57, and t. ii., p. 893) speaks of the existence of a solar atmosphere (limbus circa solem, coma lucida), which, in eclipses of the Sun, prevents it "from being quite night:" and even more uncertain, or indeed erroneous, is the assumption that the "trabes quas [Greek word] vocant" (Plin., ii., 26 and 27) had reference to the tongue-shaped rising zodiacal light, as Cassini (p. 231, art. xxxi.) and Mairan (p. 15) have maintained. Every where among the ancients the trabes are associated with the bolides (ardores et faces) and other fiery meteors, and even with long-barbed comets. (Regarding [Greek words] . see Schäfer, 'Schol. Par. ad Apoll. Rhod.', 1813, t. ii., p. 206; Pseudo-Aristot., 'de Mundo, 2, 9; 'Comment. Alex. Joh. Philop. et Olymp. in Aristot. Meteor.', lib. i., cap. vii., 3, p. 195, Ideler; Seneca, 'Nat. Quæst.', i., 1.)

We may conjecture, with much probability, that the remarkable light on the elevated plains of Mexico, seen for forty nights consecutively i8n 1509, and observed in the eastern horizon rising pyramidally from the earth, was the zodiacal light. I found a notice of this phenomenon in an ancient Aztec MS., the 'CodexTelleriano-Remensis',* preserved in the Royal Library at Paris.

[footnote] *Humboldt, 'Monumens des Peuples Indigènes de l'Amérique', t. ii., p. 301. The rare manuscript which belonged to the Archbishop of Rheims, Le Tellier, contains various kinds of extracts from an Aztec ritual, an astrological calendar, and historical annals, extending from 1197 to 1549, and embracing a notice of different natural phenomena, epochs of earthquakes and comets (as, for instance, those of 1490 and 1529), and of (which are important in relation to Mexican chronology) solar eclipses. In Camargo's manuscript 'Historia de Tlascala', the light rising in the east almost to the zenith is, singularly enough, described as "sparkling, and as if sown with stars." The description of this phenomenon, which lasted forty days, can not in any way apply to volcanic eruptions of Popcatepetl, which lies very near, in the southeastery direction. (Prescott, 'History of the Conquest of Mesico', vol. i., p. 284.) Later commentators have confounded this phenomenon, which Montezuma regarded as a warning of his misfortunes, with the "estrella que humeava" (literally, 'which spring forth'; Mexican 'choloa, to leap or spring forth'). With respect to the connection of this vapor with the star Citlal Choloha (Venus) and with "the mountain of the star" (Citialtepetl, the volcano of Orizaba), see my 'Monumens', t. ii., p. 303.

This phenomenon, whose primordial antiquity can scarcely be doubted, and which was first noticed in Europe by Childrey and Dominicus Cassini, is not the luminous solar atmosphere itself, since this can not, in accordance with mechanical laws, be more compressed than in the relation of 2 to 3, and consequently can not be diffused beyond 9/20ths of Mercury's heliocentric distance. These same laws teach us that the altitude of the extreme boundaries of the atmosphere of a cosmical p 141 body above its equator, that is to say, the point at which gravity and centrifugal force are in equilibrium, must be the same as the altitude at which a satellite would rotate round the central body simultaneously with the diurnal revolution of the latter.*

[footnote] *Laplace, 'Expos. du Syst. du Monde', p. 270; 'Mécanique Céleste', t. ii., p. 169 and 171; Schubert, 'Astr.', bd. iii., § 206.

This limitation of the solar atmosphere in its present concentrated condition is especially remarkable when we compare the central body of our system with the nucleus of other nebulous stars. Herschel has discovered several, in which the radius of the nebulous matter surrounding the star appeared at an angle of 150". On the assumption that the parallax is not fully equal to 1", we find that the outermost nebulous layer of such a star must be 150 times further from the central body than our Earth is from the Sun. If, therefore, the nebulous star were to occupy the place of our Sun, its atmosphere would not only include the orbit of Uranus, but even extend eight times beyond it.•

[footnote] *Arago, in the 'Annuaire', 1842, p. 408. Compare Sir John Herschel's considerations on the volume and faintness of light of planetary nebulæ, in Mary Somerville's 'Connection of the Physical Sciences', 1835, p. 108. The opinion that the Sun is a nebulous star, whose atmosphere presents the phenomenon of zodiacal light, did not originate with Dominicus Cassini, but was first promulgated by Mairan in 1730 ('Traité de l'Aurore Bor.', p. 47 and 263; Arago, in the 'Annuaire', 1842, p. 412). It is a renewal of Kepler's views.

Considering the narrow limitation of the Sun's atmosphere, which we have just described, we may with much probability regard the existence of a very compressed annulus of nebulous matter,* revolving freely in space between the orbits of Venus and Mars, as the material cause of the zodiacal light.

[footnote] *Cominicus Cassini was the first to assume, as did subsequently Laplace, Schubert, and Poisson, the hypothesis of a separate ring to explain the form of the zodiacal light. He says distinctly, "If the orbits of Mercury and Venus were visible (throughout their whole extent), we should invariably observe them with the same figure and in the same position with regard to the Sun, and at the same time of the year with the zodiacal light." ('Mém. de l'Acad.', t. viii., 1730, p. 218, and Biot, in the 'Comptes Rendus', 1836, t. iii., p. 666.) Cassini believed that the nebulous ring of zodiacal light consisted of innumerable small planetary bodies revolving round the Sun. He even went so far as to believe that the fall of fire-balls might be connected with the passage of the Earth through the zodiacal nebulous ring. Olmsted, and especially Biot (op. cit., p. 673), have attempted to establish its connection with the November phenomenon — a connection which Olbers doubts. (Schum., 'Jahrb.', 1837, s. 281.) Regarding the question whether the place of the zodiacal light perfectly coincides with that of the Sun's equator, see Houzeau, in Schum., 'Astr. Nachr.', 1843, No. 492, s. 190.

As p 142 yet we certainly know nothing definite regarding its actual material dimensions; its augmentation* by emanations from the tails of myriads of comets that come within the Sun's vicinity; the singular changes affecting its expansion, since it sometimes does not apper to extend beyond our Earth's orbit; or, lastly, regarding its conjectural intimate connection with the more condensed cosmical vapor in the vicinity of the Sun.

[footnote] *Sir John Herschel, 'Astron.', § 487.

The nebulous particles composing this ring, and revolving round the sun in accordance with planetary laws, may either be self-luminous or receive light from that luminary. Even in the case of a terrestrial mist (and this fact is very remarkable), which occurred at the time of the new moon at midnight in 1743, the phosphorescence was so intense that objects could be distinctly recognized at a distance of more than 600 feet.

I have occasionally been astonished in the tropical climates of south america, to observe the variable intensity of the zodiacal light. As i passed the nights, during many months, in the open air, on the shores of rivers and on ilanos, i enjoyed ample opportunities of carefully examining this phenomenon. When the zodiacal light had been most intense, i have observed that it would be perceptibly weakened for a few minutes, until it again suddenly shone forth in full brilliancy. In some few instances i have thought that i could perceive — not exactly a reddish coloration, nor the lower portion darkened in an arc-like form, nor even a scintillation, as mairan affirms he has observed — but a kind of flickering and wavering of the light.*

[footnote] *Arago, in the 'Annuaire', 1832, p. 246. Several physical facts appear to indicate that, in a mechanical separation of matter into its smallest particles, if the mass be very small in relation to the surface, the electrical tension may increase sufficiently for the production of light and heat. Experiments with a large concave mirror have not hitherto given any positive evidence of the presence of radiant heat in the zodiacal light. (Lettre de M. Matthiessen à M. Arago, in the 'Comptes Rendus', t. xvi., 1843, Avril, p. 687.)

Must we suppose that changes are actually in progress in the nebulous ring? or is it not more probable that, although I could not, by my meteorological instruments, detect any change of heat or moisture near the ground, and small stars of the fifth and sixth magnitudes appeared to shine with equally undiminished intensity of light, processes of condensation may be going on in the uppermost strata of the air, by means of which the transparency, or rather, the reflection of light, may be modified in some peculiar and unknown manner? p 143 An assumption of the existence of such meteorological causes on the confines of our atmosphere is strengthened by the "sudden flash and pulsation of light," which, according to the acute observations of Olbers, vibrated for several seconds through the tail of a comet, which appeared during the continuance of the pulsations of light to be lengthened by several degrees, and then again contracted.*

[footnote] *"What you tell me of the changes of light in the zodiacal light, and of the causes to which you ascribe such changes within the tropics, is of the greatr interest to me, since I have been for a long time past particularly attentive, every spring, to this phenomenon in our northern latitudes. I, too, have always believed that the zodiacal light rotated; but I assumed (contrary to Poisson's opinion, which you have communicated to me) that it completely extended to the Sun, with considerably augmenting brightness. The light circle which, in total solar eclipses, is seen surrounding the darkened Sun, I have regarded as the brightest portion of the zodiacal light. I have convinced my self that this light is very different in different years, often for several successive years being very bright and diffused, while in othr years it is scarcely perceptible. I tyhink that I find the first trace of an allusion to the zodiacal light in a letter from Rothmann to Tycho, in which he mentions that in the spring he has observed the twilight did not close until the sun was 24ºdegrees below the horizon. Rothmann must certainly have confounded the disappearance of the setting zodiacal light in the vapors of the western horizon with the actual cessation of twilight. I have failed to observe the pulsations of the light, probably on account of the faintness with which it appears in these countries. You are, however, certainly right in ascribing those rapid variations in the light of the heavenly bodies, which you have perceived in tropical climates, to our own atmosphere, and especially to its higher regions. This is especially in the clearest weather, that these tails exhibit pulsations, commencing from the head, as being the lowest part, and vibrating in one or two seconds through the entire tail, which thus appears rapidly to become some degrees longer, but again as rapidly contracts. That these undulations, which were formerly noticed with attention by Robert Hooke, and in more recent times by Schröter and Chladni, 'do not actually occur in the tails of the comets', but are produced by our atmosphere, is obvious when we recollect that the individual parts of those tails (which are many millions of miles in length) lie 'at very different distances' from us, and that the light from their extreme points can only reach us at intervals of time which differ several minutes from one another. Whether what you saw on the Orinoco, not at intervals of seconds, but of minutes, were actual coruscations of the zodiacal light, or whether they belonged exclusively to the upper strata of our atmosphere, I will not attempt to decide; neither can I explain the remarkable 'lightness of whole nights', nor the anomalous augmentation and prolongation of the twilight in the year 1831, particularly if, as has been remarked, the lightest part of these singular twilights did not coincide with the Sun's place below the horizon." (From a lettr written by Dr. Olbers to myself, and dated Bremen, Marth 26th, 1833.)

As, however, the separate particles of a comet's tail, measuring millions of miles, p 144 are very unequally distant from earth, it is not possible, according to the laws of the velocity and transmission of light, that we should be able, in so short a period of time, to perceive any actual changes in a cosmical body of such vast extent. There considerations in no way exclude the realith of the changes that have been observed in the emanations from the more condensed envelopes around the nucleus of a comet, nor that of the sudden irradiation of the zodiacal light, from internal molecular motion, nor of the increased or diminished reflection of light in the cosmical vapor of the luminous ring, but should simply be the means of drawing our attention to the differences existing between that which appertains to the air of heaven (the realms of universal space) and that which belongs to the strata of our terrestrial atmosphere. It is not possible, as well-attested facts prove, perfectly to explain the operations at work in the much-contested upper boundaries of our atmosphere. The extraordinary lightness of whole nights in the year 1831, during which small print might be read at midnight in the latitudes of Italy and the north of Germany is a fact directly at variance with all that we know, according to the most recent and acute researches on the crepuscular theory, and of the height of the atmosphere.*

[footnote] *Biot, 'Traité d'Astron. Physique', 3ème éd., 1841, t. i., p. 171, 238 and 312.

The phenomena of light depend upon conditions still less understood, and their variability at twilight, as well as in the zodiacal light, excite our astonishment.

We have hitherto considered that which belongs to our solare system — that world of material forms governed by the Sun — which includes the primary and secondary planets, comets of short and long periods of revolution, meteoric asteroids, which move thronged together in streams, either sporadically or in closed rings, and finally a luminous nebulous ring, that revolves round the Sun in the vicinity of the Earth, and for which, owing to its position, we may retain the name of zodiacal light. Every where the law of periodicity governs the motions of these bodies, however different may be the amount of tangential velocity, or the quantity of their agglomerated material parts; the meteoric asteroids which enter our atmosphere from the external regions of universal space are alone arrested in the course of their planetary revolution, and retained within the sphere of a larger planet. In the solar system, whose boundaries determine the attractive force of the central body, comets are made to revolve in their elliptical p 145 orbits at a distance 44 times greater than that of Uranus; may, in those comets whose nucleus appears to us, from its inconsiderable mass, like a mere passing cosmical cloud, the Sun exercises its attractive force on the outermost parts of the emanations radiating from the tail over a space of many millions of miles. Central forces, therefore, at once constitute and maintain the system.

Our Sun may be considered as at rest when compared to all the large and small, dense and almost vaporous cosmical bodies tht appertain to and revolve around it; but it actually rotates around the common center of gravity of the whole system, which occasionally falls within itself, that is to say, remains within the material circumference of the Sun, whatever changes may be assumed by the position of the planets. A very different phenomenon is that presented by the translatory motion of the Sun, that is, the progressive motion of the center of gravity of the whole solar system in universal space. Its velocity is such* that, according to Bessel, the relative motion of the Sun, and that of 61 Cygni, is not less in one day than 3,336,000 geographical miles.

[footnote] *Bessel, in Schum., 'Jahrb. für' 1839, s. 51; probably four millions of miles daily, in a relative velocity of at the least 3,336,000 miles, or more than couble the velocity of revolution of the Earth in her orbit round the Sun.

This change of the entire solar system would remain unknown to us, if the admirable exactness of our astronomical instruments of measurement, and the advancement recently made in the art of observing, did not cause our advance toward remote stars to be perceptible, like an approximation to the objects of a distant shore in apparent motion. The proper motion of the star 61 Cygni, for instance, is so considerable, that it has amounted to a whole degree in the course of 700 years.

The amount or quantity of these alterations in the fixed stars (that is to say, the changes in the relative position of self-luminous stars toward each other), can be determined with a greater degree of certainty than we are able to attach to the genetic explanation of the phenomenon. After taking into consideration what is due to the precession of the equinoxes, and the nutation of the earth's axis produced by the action of the Sun and Moon on the spheroidal figure of our globe, and what may be ascribed to the transmission of light, that is to say, to its aberration, and to the parallax formed by the diametrically opposite position of the Earth in its course round the Sun, we still find that there is a residual portion p 146 of the annual motion of the fixed stars due to the translation of the whole solar system in universal space, and to the true proper motion of the stars. The difficult problem of numerically separating these two elements, the true and the apparent motion, has been effected by the careful study of the direction of the motion of certain individual stars, and by the consideration of the fact that, if all the stars were in a state of absolute rest, they would appear perspectively to recede from the point in space toward which the Sun was directing its course. But the ultimate result of this investigation, confirmed by the calculus of probabilities, is, that our solar system and the stars both change their places in space. According to the admirable researches of d'Argelander at Abo, who has extended and more perfectly developed the work begun by William Herschel and Prevost, the Sun moves in the direction of the constellation Hercules, and probably, from the combination of the observations made of 537 stars, toward a point lying (at the equinox of 1792.5) at 257ºdegrees 49.'7 R.A., and 28ºdegrees 49.'7 N.D. It is extremely difficult, in investigations of this nature, to separate the absolute from the relative motion, and to determine what is aloone owing to the solar system.*

[footnote] *Regarding the motion of the solar system, according to Bradley, Tobias Mayer, Lambert, Lalande, and William Herschel, see Arago in the 'Annuaire', 1842, p. 388-399' Argelander, in Schum., 'Astron. Nachr ., No. 363, 364, 398, and in the treatise 'Von der eigenen Bewegung des Sonnensystems' (On the proper Motion of the Solar System), 1837, s. 43, respecting Perseus as the central body of the whole stellar stratum, likewise Otho Struve, in the 'Bull. de l'Acad. de St. Pétersb.', 1842, t. x., No. 9, p. 137-139. The last-named astronomer has found, by a mo4re recent combination, 261ºdegrees 23' R.A.+37ºdegrees 36' Decl. for the direction of the Sun's motion; and, taking the mean of his own results with that of Argelander, we have, by a combination of 797 stars, the formula 259ºdegrees 9' R.A.+34ºdegrees 36' Decl.

If we consider the proper, and not the perspective motions of the stars, we shall find many that appear to be distributed in groups, having an opposite direction; and facts hitherto observed do not, at any rate, render it a necessary assumption that all parts of our starry stratum, or the whole of the stellar islands filling space, should move round one large unknown luminous or non-luminous central body. The tendency of the human mind to investigate ultimate and highest causes certainly inclines the intellectual activity, no less than the imagination of mankind, to adopt such an hypothesis. Even the Stagirite proclaimed that "every thing which is moved must be referable to a motor, and that there would be no end to p 147 the concatenation of causes if there were not one primordial immovable morot."*

[footnote] *Aristot., 'de Cælo', iii., 2, p. 301, Bekker: 'Phys.', viii., t, p. 256.

This material taken from pages 147-203

COSMOS: A Sketch of the Physical Description of the Universe, Vol. 1 by Alexander von Humboldt

Translated by E C Otte

from the 1858 Harper & Brothers edition of Cosmos, volume 1 —————————————————————————

The manifold translatory changes of the stars, not those produced by the parallaxes at which they are seen from the changing position of the spectator, but the true changes constantly going on in the regions of space, afford us incontrovertible evidence of the 'dominion of the laws of attraction' in the remotest regions of space, beyond the limits of our solar system. The existence of these laws is revealed to us by many phenomena, as, for instance, by the motion of double stars, and by the amount of retarded or accelerated motion in different parts of their elliptic orbits. Human inquiry need no longer pursue this subject in the domain of vague conjecture, or amid the undefined analogies of the ideal world; for even here the progress made in the method of astronomical observations and calculations has enabled astronomy to take up its position on a firm basis. It is not only the discovery of the astounding numbers of double and multiple stars revolving round a center of gravity lying 'without' their system (2800 such systems having been discovered up to 1837), but rather the extension of our knowledge regarding the fundamental forces of the whole material world, and the proofs we have obtained of the universal empire of the laws of attraction, that must be ranked among the most brilliant discoveries of the age. The periods of revolution of colored stars present the greatest differences; thus, in some instances, the period extends to 43 years, as in πpi of Corona, and in others to several thousands,, as in 66 of Cetus, 38 of Gemini, and 100 of Pisces. Since Herschel's measurements in 1782, the satellite of the nearest star in the triple system of [Greek letter] of Cancer has completed more than one entire revolution. By a skillful combination of the altered distances and angles of position,* the elements of these orbits may be found, conclusions drawn regarding the absolute distance of the double stars from the Earth, and comparisons made between their mass and that of the Sun.

[footnote] *Savary, in the 'Connaissance des Tems', 1830, p. 56 and 163. Encke, 'Berl. Jahrb.', 1832, s. 253, etc. Arago, in the 'Annuaire' 1834, p. 260, 295. John Herschel, in the 'Memoirs of the Astronom. Soc.', vol. v., p. 171.

Whether, however, here and in our solar system, quantity of matter is the only standard of the amount of attractive force, or whether 'specific' forces of attraction proportionate to the mass may not at the same time come into operation, as Bessel was the first to conjecture, are questions p 148 whose practical solution must be left to future ages.*

[footnote] * Bessel, 'Untersuchung. des Theils der planetarischen Storungen, welche aus der Bewegung der Sonne entstchen' (An Investigation of the portion of the Planetary Disturbances depending on the motion of the Sun) in 'Abh. der Berl. Akad. der Wissensch.', 1824 (Mathem. Classe), s. 2-6. The question has been raised by John Tobias Mayer, in 'Comment. Soc. Reg. Gotting.', 1804-1808, vol. xvi., p. 31-68.

When we compare our Sun with the other fixed stars, that is, with other self-luminous Suns in the lenticular starry stratum of which our system forms a part, we find, at least in the case of some, that channels are opened to us, which may lead, at all events, to an 'approximate' and limited knowledge of their relative distances, volumes, and masses, and of the velocities of their translatory motion. If we assume the distance of Uranus from the Sun to be nineteen times that of the Earth, that is to say, nineteen times as great as that of the Sun from the Earth, the central body of our planetary system will be 11,900 times the distance of Uranus from the star 'a' in the constellation Centaur, almost 31,300 from 61 Cygni, and 41,600 from Vega in the constellation Lyra. The comparison of the volume of the Sun with that of the fixed stars of the first magnitude is dependent upon the apparent diameter of the latter bodies — an extremely undertain optical element. If even we assume, with Herschel, that the apparent diameter of Arcturus is only a tenth part of a second, it still follows that the true diameter of this star is eleven times greater than that of the Sun.*

[footnote] *'Philos. Trans.' for 1803, p. 225. Arago, in the 'Annuaire', 1842, p. 375. In order to obtain a clearer idea of the distances ascribed in a rather earlier part of the text to the fixed stars, let us assume that the Earth is a distance of one foot from the Sun; Uranus is then 19 feet, and Vega Lyrae is 158 geographical miles from it.

The distance of the star 61 Cygni, made known by Bessel, has led approximately to a knowledge of the quantity of matter contained in this body as a double star. Notwithstanding that, since Bradley's observations, the portion of the apparent orbit traversed by this star is not sufficiently great to admit of our arriving with perfect exactness at the true orbit nd the major axis of this star, it has been conjectured with much probability by the great Konigsberg astronomer,* "that the mass of this double star can not be very considerably larger or smaller than half of the mass of the Sun."

[footnote] *Bessel, in Schum., 'Jahrb.', 1839, s. 53.

This result is from actual measurement. The analogies deduced from the relatively larger mass of those planets in our solar system that are attended by satellites, and from the fact that Struve has discovered six times more double stars among p 194 the brighter than among the telescopic fixed stars, have led other astronomers to conjecture that the average mass of the larger number of the binary stars exceeds the mass of the Sun.*

[footnote] *Mädler, 'Astron.', s. 476; also in Schum, 'Jahrb.', 1839, s. 95.

We are, however, far from having arrived at general results regarding this subject. Our Sun, according to Argenlander, belongs, with reference to proper motion in space, to the class of rapidly-moving fixed stars.

The aspect of the starry heavens, the relative position of stars and nebullae, the distribution of their luminous masses, the picturesque beauty, if I may so express myself, of the whole firmament, depend in the course of ages conjointly upon the proper motion of the stars and nebulae, the translation of our solar system in space, the appearance of new stars, and the disappearance or sudden diminution in the intensity of the light of others, and lastly and specially, on the changes which the Earth's axis experiences from the attraction of the Sun and Moon. The beautiful stars in the constellation of the Centaur and the Southern Cross will at some future time be visible in our northern latitudes, while other stars, as Sirius and the stars in the Belt of Orion, will in their turn disappear below the horizon. The places of the North Pole will successively be indicated by the stars ß beta and a alpha Cephei, and ∂ delta Cygni, until after a period of 12,000 years, Vega in Lyra will shine forth as the brightest of all possible pole stars. These data give us some idea of the extent of the motions which, divided into infinitely small portions of time, proceed without intermission in the great chronometer of the universe. If for a moment we could yield to the power of fancy, and imagine the acuteness of our visual organs to be made equal with the extremest bounds of telescopic vision, and bring together that which is now divided by long periods of time, the apparent rest that reigns in space would suddenly disappear. We should see the countless host of fixed stars moving in thronged groups in different directions; nebulae wandering through space, and becoming condensed and dissolved like cosmical clouds; the vail of the Milky Way separated and broken up in many parts, and 'motion' ruling supreme in every portion of the vault of heave, even as on the Earth's surface, where we see it unfolded in the germ, the leaf, and the blossom, the organisms of the vegetable world. The celebrated Spanish botanist Cavanilles was the first who entertained the idea of "seeing grass grow," and he directed the horizontal micrometer threads of a powerfully magnifying glass at one time to p 150 the apex of the shoot of a bambusa, and at another on the rapidly-growing stem of an American aloe ('Agave Americana', precisely as the astronomer places his cross of net-work against a culminating star. In the collective life of physical nature, in the organic as in the sidereal world, all things that have been, that are, and will be, are alike dependent on motion.

The breaking up of the Milky Way, of which I have just spoken, requires special notice. William Herschel, our safe and admirable guide to this portion of the regions of space, has discovered by his star-guagings that the telescopic breadth of the Milky Way extends from six to seven degrees beyond what is indicated by our astronomical maps and by the extent of the sidereal radiance visible to the naked eye.*

[footnote] *Sir William Herschel, in the 'Philos. Transact.' for 1817, Part ii p. 438.

The two brilliant nodes in which the branches of the zone unite, in the region of Cepheus and Cassiopeia, and in the vicinity of Scorpio and Sagittarius, appear to exercise a powerful attraction on the contiguous stars; in the most brilliant part, however between beta and [Greek symbol] Cygni, one half of the 330,000 stars that have been discovered in a breadth of 5 degrees are directed toward one side, and the remainder to the other. It is in this part that Herschel supposes the layer to be broken up.*

[footnote] *Arago, in the 'Annuaire', 1842, p. 569

The number of telescopic stars in the Milky Way uninterrupted by any nebulae is estimated at 18 millions. In order, I will not say, to realize the greatness of this number, but, at any rate, to compare it with something analogous, I will call attention to the fact that there are not in the whole heavens more than about 8000 stars between the first and the sixth magnitudes, visible to the naked eye. The barren astonishment excited by numbers and dimensions in space, when not considered with reference to applications engaging the mental and perceptive powers of man, is awakened in both extremes of the universe, in the celestial bodies as in the minutest animalcules.*

[footnote] *Sir John Herschel, in a letter from Feldhuysen, dated Jan. 13th, 1836. Nicholl, 'Architecture of the Heavens', 1838, p. 22. (See, also, some separate notices by Sir William Herschel on the starless space which separates us by a great distance from the Milky Way, in the 'Philos. Transact.' for 1817, Part ii., p. 328.)

A cubic inch of the polishing slate of Bilin contains, according to
Ehrenberg, 40,000 millions of the silicious shells of Galionellae.

The stellar Milky Way, in the region of which, according to Argelander's admirable observations, the brightest stars of the firmament appear to be congregated, is almost at right angles p 151 with another Milky Way, composed of nebulae. The former constitutes, according to Sir John Herschel's views, an annulus, that is to say, an independent zone, somewhat remote from our lenticular-shaped starry stratum, and similar to Saturn's ring. Our planetary system lies in an eccentric direction, nearer to the region of the Cross than to the diametrically opposite point, Cassiopeia.*

[footnote] *Sir John Herschel, 'Astronom.', 624; likewise in his 'Observations on Nebulae and Clusters of Stars' ('Phil. Transact.', 1833, Part ii., p. 479, fig. 25): "We have here a brother system, bearing a real physical resemblance and strong analogy of structure to our own."

An imperfectly seen nebulous spot, discovered by Messier in 1774, appeared to present a remarkable similarity to the form of our starry stratum and the divided ring of our Milky Way.*

[footnote] *Sir William Herschel, in the 'Phil. Trans.' for 1785, Part i., p. 257. Sir John Herschel, 'Astron.', 616. ("The 'nebulous' region of the heavens forms 'a nebulous Milky Way', composed of distinct nebulae, as the other of stars." The same observation was made in a letter he addressed to me in March, 1829.)

The Milky Way composed of nebulae does not belong to our starry stratum, but surrounds it at a great distance without being physically connected with it, passing almost in the form of a large cross through the dense nebulae of Virgo, especially in the northern wing, through Comae Berenicis, Ursa Major, Andromeda's girdle, and Pisces Boreales. It probably intersects the stellar Milky Way in Cassiopeia, and connects its dreary poles (rendered starless from the attractive forces by which stellar bodies are made to agglomerate into groups) in the least dense portion of the starry stratum.

We see from these considerations that our starry cluster, which bears traces in its projecting branches of having been subject in the course of time to various metamorphoses, and evinces a tendency to dissolve and separate, owing to secondary centers of attraction — is surrounded by two rings, one of which, the nebulous zone, is very remote, while the other is nearer, and composed of stars alone. The latter, which we generally term the Milky Way, is composed of nebulous stars, averaging from the tenth to the eleventh degree of magnitude,* but appearing, when considered individually, of very different magnitudes, while isolated starry clusters (starry swarms) almost always exhibit throughout a character of great uniformity in magnitude and brilliancy.

[footnote] *Sir John Herschel, 'Astron.', 585.

In whatever part the vault of heaven has been pierced by powerful and far-penetrating telescopic instruments, stars or luminous nebulae are every where discoverable, the former, in p 152 some cases, not exceeding the twentieth or twenty-fourth degree of telescopic magnitude. A portion of the nebulous vapor would probably be found resolvable into stars by more powerful optical instruments. As the retina retains a less vivid impression of separate than of infinitely near luminous points, less strongly marked photometric relations are excited in the latter case, as Arago has recently shown.*

[footnote] *Arago, in the 'Annuaire', 1842, p. 282-285, 409-411, and 439-442.

The definite or amorphous cosmical vapor so universally diffused, and which generates heat through condensation, probably modifies the transparency of the universal atmosphere, and diminishes that uniform intensity of light which, according to Halley and Olbers, should arise, if every point throughout the depths of space were filled by an infinite series of stars.*

[footnote] *Olbers, on the transparency of celestial space, in Bode's 'Jahrb.', 1826, s. 110-121.

The assumption of such a distribution in space is, however, at variance with observation, which shows us large starless regions of space, 'openings' in the heavens, as William Herschel terms them — one, four degrees in width, in Scorpio, and another in Serpentarius. In the vicinity of both, near their margin, we find unresolvable nebulae, of which that on the western edge of the opening Scorpio is one of the most richly thronged of the clusters of small stars by which the firmament is adorned. Herschel ascribes these openings or starless regions to the attractive and agglomerative forcesof the marginal groups.*

[footnote] *"An opening in the heavens," William Herschel, in the 'Phil. Trans.' for 1785, vol. lxxv., Part i., p. 256. Le Francais Lalande, in the 'Connaiss. des Tems pour l'An.' VIII., p. 383. Arago, in the 'Annuaire', 1842, p. 425.

"They are parts of our starry stratum," says he, with his usual graceful animation of style, "that have experienced great devastation from time." If we picture to ourselves the telescopic stars lying behind one another as a starry canopy spread over the vault of heaven, these starless regions in Scorpio and Serpentarius may, I think, be regarded as tubes through which we may look into the remotest depths of space. Other stars may certainly lie in those parts where the strata forming the canopy are interrupted, but these are unattainable by our instruments. The aspect of fiery meteors had led the ancients likewise to the idea of clefts or openings ('chasmata') in the vault of heaven. These openings were, however, only regarded as transient, while the reason of their being luminous and fiery, instead of obscure, was supposed to be owing to the p 153 translucent illuminated ether which lay beyond them.*

[footnote] *Aristot., 'Meteor.', ii.,, 5, 1. Seneca, 'Natur. Quaest.', i., 14, 2. "Coelum discessisse," in Cic., 'de Divin.', i., 43.

Derham, and even Huygens, did not appear disinclined to explain in a similar manner the mild radiance of the nebulae.*

[footnote] *Arago, in the 'Annuaire', 1842, p. 429.

When we compare the stars of the first magnitude, which, on an average, are certainly the nearest to us, with the non-nebulous telescopic stars, and further, when we compare the nebulous stars with unresolvable nebulae, for instance, with the nebula in Andromeda, or even with the so-called planetary nebulous vapor, a fact is made manifest to us by the consideration of the varying distances and the boundlessness of space, which shows the world of phenomena, and that which constitutes its causal reality, to be dependent upon the 'propagation of light'. The velocity of this propagation is according to Struve's most recent investigations, 166,072 geographical miles in a second, consequently almost a million of times greater than the velocity of sound. According to the measurements of Maclear, Bessel, and Struve, of the parallaxes and distances of three fixed stars of very unequal magnitudes ('a' Centauri, 16 Cygni, and 'a' Lyrae), a ray of light requires respectively 3, 9 1/4, and 12 years to reach us from these three bodies. In the short but memorable period between 1572 and 1604, from the time of Cornelius Gemma and Tycho Brahe to that of Kepler, three new stars suddenly appeared in Cassiopeia and Cygnus, and in the foot of Serpentarius. A similar phenomenon exhibited itself at intervals in 1670, in the constellation Vulpis. In recent times, even since 1837, Sir John Herschel has observed, at the Cape of Good Hope, the brilliant star [Greek symbol] in Argo increase in splendor from the second to the first magnitude.*

[footnote] *In December, 1837, Sir John Herschel saw the star [Greek symbol] Argo, which till that time appeared as of the second magnitude, and liable to no change, rapidly increase till it became of the first magnitude. In January, 1838, the intensity of its light was equal to that of 'a' Centauri. According to our latest information, Maclear in March, 1843, found it as bright as Canopus; and even 'a' Crucis looked faint by [Greek symbol] Argo.

These events in the universe belong, however, with reference to their historical reality, to other periods of time than those in which the phenomena of light are first revealed to the inhabitants of the Earth: they reach us like the voices of the past. It has been truly said, that with our large and powerful telescopic instruments we penetrate alike through the boundaries of time and space: we measure the former through the latter, for in the course of an p 154 hour a ray of light traverses over a space of 592 millions of miles. While according to the theogony of Hesiod, the dimensions of the universe were supposed to be expressed by the time occupied by bodies in falling to the ground ("the brazen anvil was not more than nine days and nine nights in falling from heaven to earth"), the elder Herschel was of opinion* that light required almost two millions of years to pass to the Earth from the remotest luminous vapor reached by his forty-foot reflector.

[footnote] *"Hence it follows that the rays of light of the remotest nebulae must have been almost two millions of years on their way, and that consequently, so many years ago, this object must already have had an existence in the sidereal heaven, in order to send out those rays by which we now perceive it." William Herschel, in the 'Phil. Trans.' for 1802, p. 498. John Herschel, 'Astron.', 590. Arago, in the 'Annuaire', 1842, p. 334, 359, and 382-385.

Much, therefore, has vanished long before it is rendered visible to us — much that we see was once differently arranged from what it now appears. The aspect of the starry heavens presents us with the spectacle of that which is only apparently simultaneous, and however much we may endeavor, by the aid of optical instruments, to bring the mildly-radiant vapor of nebulous masses or the faintly-glimmering starry clusters nearer, and diminish the thousands of years interposed between us and them, that serve as a criterion of their distance, it still remains more than probable, from the knowledge we possess of the velocity of the transmission of luminous rays, that the light of remote heavenly bodies presents us with the most ancient perceptible evidence of the existence of matter. It is thus that the reflective mind of man is led from simple premises to rise to those exalted heights of nature, where in the light-illumined realms of space, "myriads of worlds are bursting into life like the grass of the night."*

[fotnote] *From my brother's beautiful sonnet "Freiheit und Gesetz." (Wilhelm von Humboldt, 'Gesammelte Werke', bd. iv., s. 358, No. 25.)

From the regions of celestial forms, the domain of Uranus, we will now descend to the more contracted sphere of terrestrial forces — to the interior of the Earth itself. A mysterious chain links together both classes of phenomena. According to the ancient signification of the Titanic myth,* the powers of organic life, that is to say, the great order of nature, depend upon the combined action of heaven and earth.

[footnote] *Otfried Muller, 'Prolegomena', s. 373.

If we suppose that the Earth, like all the other planets, primordially belonged, according to its origin, to the central body, the Sun, and to the solar atmosphere that has been separated into nebulous p 155 rings, the same connection with this continguous Sun, as well as with all the remote suns that shine in the firmament, is still revealed through the phenomena of light and radiating heat. The difference in the degree of these actions must not lead the physicist, in his delineation of nature, to forget the connection and the common empire of similar forces in the universe. A small fraction of telluric heat is derived from the regions of universal space in which our planetary system is moving, whose temperature (which according to Fourier, is almost equal to our mean icy polar heat) is the result of the combined radiation of all the stars. The causes that more powerfully excite the light of the Sun in the atmosphere and in the upper strata of our air, that give rise to heat-engendering electric and magnetic currents, and awaken and genially vivify the vital spark in organic structures on the earth's surface, must be reserved for the subject of our future consideration.

As we purpose for the present to confine ourselves exclusively within the telluric sphere of nature, it will be expedient to cast a preliminary glance over the relations in space of solids and fluids, the form of the Earth, its mean density, and the partial distribution of this density in the interior of our planet, its temperature and its electro-magnetic tension. From the consideration of these relations in space, and of the forces inherent in matter, we shall pass to the reaction of the interior on the exterior of our globe; and to the special consideration of a universally distributed natural power — subterranean heat; to the phenomena of earthquakes, exhibited in unequally expanded circles of commotion, which are not referable to the action of dynamic laws alone; to the springing forth of hot wells; and, lastly, to the more powerful actions of volcanic processes. The crust of the Earth, which may scarcely have been perceptibly elevated by the sudden and repeated, or almost uninterrupted shocks by which it has been moved from below, undergoes, nevertheless, great changes in the course of centuries in the relations of the elevation of solid portions, when compared with the surface of the liquid parts, and even in the form of the bottom of the sea. In this manner simultaneous temporary or permanent fissures are opened, by which the interior of the Earth is brought in contact with the external atmosphere. Molten masses, rising from an unknown depth, flow in narrow streams along the declivity of mountains, rushing impetuously onward, or moving slowly and gently, until the fiery source is quenched in the midst of exhalations, and the lava becomes incrusted, as it were, by p 156 the solidification of its outer surface. New masses of rocks are thus formed before our eyes, while the older ones are in their turn converted into other forms by the greater or lesser agency of Platonic forces. Even where no disruption takes place the crystalline moleculres are displaced, combining to form bodies of denser texture. The water presents structures of a totally different nature, as, for instance, concretions of animal and vegetable remains, of earthy, calcareous, or aluminous precipitates, agglomerations of finely-pulverized mineral bodies, covered with layers of the silicious shields of infusoria, and with transported soils containing the bones of fossil animal forms of a more ancient world. The study of the strata which are so differently formed and arranged before our eyes, and of all that has been so variously dislocated, conforted, and upheaved, by mutual compression and volcanic force, leads the reflective observer, by simple analogies, to draw a comparison between the present and an age that has long passed. It is by a combination of actual phenomena, by an ideal enlargement of relations in space, and of the amount of active forces, that we are able to advance into the long sought and indefinitely anticipated domain of geognosy, which has only within the last half century been based on the solid foundation of scientific deduction.

It has been acutely remarked, "that notwithstanding our continual employment of large telescopes, we are less acquainted with the exterior than with the interior of other planets, excepting, perhaps, our own satellite." They have been weighed, and their volume measured; and their mass and density are becoming known with constantly-increasing exactness; thanks to the progress made in astronomical observation and calculation. Their physical character is, however, hidden in obscurity, for it is only in our own globe that we can be brought in immediate contact with all the elements of organic and inorganic creation. The diversity of the most heterogenous substances, their admixtures and metamorphoses, and the ever-changing play of the forces called into action, afford to the human mind both nourishment and enjoyment, and open an immeasurable field of observation, from which the intellectual activity of man derives a great portion of its grandeur and power. The world of perceptive phenomena is reflected in the depths of the ideal world, and the richness of nature and the mass of all that admits of classification gradually become the objects of inductive reasoning.

I would here allude to the advantage, of which I have already p 157 spoken, possessed by that portion of physical science whose origin is familiar to us, and is connected with our earthly existence. The physical description of celestial bodies from the remotely-glimmering nebulae with their suns, to the central body of our own system, is limited, as we have seen, to general conceptions of the volume and quantity of matter. No manifestation of vital activity is there presented to our senses. It is only from analogies, frequently from purely ideal combinations, that we hazard conjectures on the specific elements of matter, or on their various modifications in the different planetary bodies. But the physical knowledge of the heterogeneous nature of matter, its chemical differences, the regular forms in which its molecules combine together, whether in crystals or granules; its relations to the deflected or decomposed waves of light by which it is penetrated; to radiating, transmitted, or polarized heat; and to the brilliant or invisible, but not, on that account, less active phenomena of electro-magnetism — all this inexhaustible treasure, by which the enjoyment of the contemplation of nature is so much heightened, is dependent on the surface of the planet which we inhabit, and more on its solid than on its liquid parts. I have already remarked how greatly the study of natural objects and forces, and the infinite diversity of the sources they open for our consideration, strengthen the mental activity, and call into action every manifestation of intellectual progress. These relations require, however, as little comment as that concatenation of causes by which particular nations are permitted to enjoy a superiority over others in the exercise of a material power derived from their command of a portion of these elementary forces of nature.

If, on the one hand, it were necessary to indicate the difference existing between the nature of our knowledge of the Earth and of that of the celestial regions and their contents, I am no less desirous, on the other hand, to draw attention to the limited boundaries of that portion of spacefrom which we derive all our knowledge of the heterogeneous character of matter. This has been somewhat inappropriately termed the Earth's crust; it includes the strata most contiguous to the upper surface of our planet, and which have been laid open before us by deep fissure-like valleys, or by the labors of man, in the bores and shafts formed by miners. These labors* do not extend beyond a vertical depth of somewhat more than 2000 feet (about one third of a geographical mile) below the p 159 level of the sea, and consequently only about 1/9800th of the Earth's radius.

[footnote] *In speaking of the greatest depths within the Earth reached by human labor, we must recollect that there is a difference between the 'absolute depth' (that is to say, the depth below the Earth's surface at that point) and the 'relative depth' (or that beneath the level of the sea). The greatest relative depth that man has hitherto reached is probably the bore at the new salt-works at Minden, in Prussia: in June, 1814, it was exactly 1993 feet, the absolute depth being 2231 feet. The temperature of the water at the bottom was 98 degrees F., which assuming the mean temperature of the air at 49.3 degrees gives an augmentation of temperature of 1 degree for every 54 feet. The absolute depth of the Artesian well of Grenelle, near Paris, is only 1795 feet. According to the account of the missionary Imbert, the fire-springs, "Ho-tsing." of the Chinese, which are sunk to obtain [carbureted] hydrogen gas for salt-boiling, far exceed our Artesian springs in depth. In the Chinese province of Szu-tschuan these fire-springs are very commonly of the depth of more than 2000 feet; indeed, at Tseu-lieu-tsing (the place of continual flow) there is a Ho-tsing which, in the year 1812, was found to be 3197 feet deep. (Humboldt, 'Asie Centrale', t. ii., p. 521 and 525. 'Annales de l'Association de la Propagation de la Foi', 1829, No. 16, p. 369.)

[footnote continues] The relative depth reached at Mount Massi, in Tuscany, south of Volterra, amounts, according to Matteuci, to only 1253 feet. The boring at the new salt-works near Minden is probably of about the same relative depth as the coal-mine at Apendale, near Newcastle-under-Lyme, in Staffordshire, where men work 725 yards below the surface of the earth. (Thomas Smith, 'Miner's Guide', 1836, p. 160.) Unfortunately, I do not know the exact height of its mouth above the level of the sea. The relative depth of the Monk-wearmouth mine, near Newcastle, is only 1496 feet. (Phillips, in the 'Philos. Mag.', vol. v., 1834, p. 446.) That of the Liege coal-mine, 'l'Esperance' at Seraing, is, according to M. Gernaert, Ingenieur des Mines, 1223 feet in depth. The works of greatest absolute depth that have ever been formed are for the most part situated in such elevated plains or valleys that they either do not descend so low as the level of the sea, or at most reach very little below it. Thus the Eselchacht, at Kuttenberg, in Bohemia, a mine which can not now be worked, had the enormous absolute depth of 3778 feet. (Fr. A. Schmidt, 'Berggestze der oter Mon.', abth. i., bd. i., s. xxxii.) Also, at St. Daniel and at Geish, on the Rorerbubel, in the 'Landgericht' (or provincial district) of Kitzbuhl, there were, in the sixteenth century, excavations of 3107 feet. The plans of the works of the Rorerbubel are still preserved. (See Joseph von Sperges, 'Tyroler Bergwerksgeschichte', s. 121. Compare, also, Humboldt, 'Gutachten uber √∫erantreibung des Meissner Stollens in die Freiberger Erzrevier', printed in Herder, 'uber Herantreibung des Meissner Stollens in die Freiberger Erzrevier', printed in Herder, 'uber den jetz begonnenen Erbstollen', 1838, s. cxxiv.) We may presume that the knowledge of the extraordinary depth of the Rorerbuhel reached England at an early period, for I find it remarked in Gilbert, 'de Magnete', that men have penetrated 2400 or even 3000 feet into the crust of the Earth. ("Exigua videtur terrae portio, quae unquam hominibus spectanda emerget aut eruitur; cum profundinus in ejus viscera, ultra efflorescentis extremitatis corruptelam, aut propter aquas in magnis fodin, tanquam per venas scaturientesaut propter seris salubrioris ad vitam operariorum sustinendam necessarii defectum, aut propter ingentex sumptus ad tantos labores exantlandos, multasque difficultates, ad profundiores terrz' partes penetrre non possumus; adeo ut quadrigentas aut [quod rarissime] quingentas orgyas in quibusdam metallis descendisse, stupendus omnibus videatur connatus." — Guilielmi Gilberti, Colcestrensis, 'de Magnete Physiologia nova'. Lond., 1600, p. 40.)

[footnote continues] The absolute depth of the mines in the Saxon Erzgebirge, near Freiburg, are: in the Thurmhofer mines, 1944 feet; in the Honenbirker mines, 1827 feet; the relative depths are only 677 and 277 feet, if, in order to calculate the elevation of the mine's mouth above the level of the sea, we regard the elevation of Freiburg as determined by Reich's recent observations to be 1269 feet. The absolute depth of the celebrated mine of Joachimsthal, in Bohemia (Verkreuzung des Jung Hauer Zechen-und Andreasganges), is full 2120 feet; so that, as Von Dechen's measurements show that its surface is about 2388 feet above the level of the sea, it follows that the excavations have not as yet reached that point. In the Harz, the Samson mine at Andreasberg has an absolute depth of 2197 feet. In what was formerly Spanish America, I know of no mine deeper than the Valenciana, near Guanaxuato (Mexico), where I found the absolute depth of the Planes de San Bernardo to be 1686 feet; but these planes are 5960 feet above the level of the sea. If we compare the depth of the old Kuttenberger mine (a depth greater than the height of our Brocken, and only 200 feet less than that of Vesuvius) with the loftiest structures that the hands of man have erected (with the Pyramid of Cheops and with the Cathedral of Strasburg), we find that they stand in the ratio of eight to one. In this note I have collected all the certain information I could find regarding the greatest absolute and relative depths of mines and borings. In descending eastward from Jerusalem toward the Dead Sea, a view presents itself to the eye, which, according to our present hypsometrical knowledge of the surface of our planet, is unrivaled in any country; as we approach the open ravine through which the Jordan takes its course, we tread, with the open sky above us, on rocks which, according to the barometric measurements of Berton and Russegger are 1385 feet below the level of the Mediterranean. (Humboldt, 'Asie Centrale', th. ii., p. 323.)

The crystalline masses that have been erupted from active volcanoes, and are generally similar to the rocks on the upper surface, have come from depths which, although not accurately determined, must certainly be sixty times greater than those to which human labor has been enabled to penetrate. We are able to give in numbers the depth of the shaft where the strata of coal, after penetrating a certain way, rise again at a distance that admits of being accurately defined by measurements. These dips show that the carboniferous strata, together with the fossil organic remains which they contain, must lie, as, for instance, in Belgium, more than five or six thousand feet* below the present level p 160 of the sea, and that the calcareous and the curved strata of the Devonian basin penetrate twice that depth.

[footnote] *Basin-shaped curved strata, which dip and reappear at measureable distances, although their deepest portions are beyond the reach of the miner, afford sensible evidence of the nature of the earth's crust at great depths below its surface. Testimony of this kind possesses, consequently, a great geognostic interest. I am indebted to that excellent geognosist, Von Dechen, for the following observations. "The depth of the coal basin of Liege, at Mont St. Gilles, which I, in conjunction with our friend Von Oeynhausen, have ascertained to be 3890 feet below the surface, extends 3464 feet below the surface of the sea, for the absolute height of Mont St. Gilles certainly does not much exceed 400 feet; the coal basin of Mons is fully 1865 feet deeper. But all these depths are trifling compared with those which are presented by the coal strata of Saar-Revier (Saarbrucken). I have found after repeated examinations, that the lowest coal stratum which is known in the neighborhood of Duttweiler, near Bettingen, northeast of Saarlouis, must descend to depths of 20,682 and 22,015 feet (or 3.6 geographical miles) below the level of the sea." This result exceeds, by more than 8000 feet, the assumption made in the text regarding the basin of the Devonian strata. This coal-field is therefore sunk as far below the surface of the sea as Chimborazo is elevated above it — at a depth at which the Earth's temperature must be as high as 435ºdegrees F. Hence, from the highest pinnacles of the Himalaya to the lowest basins containing the vegetation of an earlier world, there is a vertical distance of about 48,000 feet, or of the 435th part of the Earth's radius.

If we compare these subterranean basins with the summits of montains that have hitherto been considered as the most elevated portions of the raised crust of the Earth, we obtain a distance of 37,000 feet (about seven miles), that is, about the 1/524th of the Earth's radius. These, therefore, would be the limits of vertical depth and of the superposition of mineral strata to which geognostical inquiry could penetrate, even if the general elevation of the upper surface of the earth were equal to the height of the Dhawalagigi in the Himalaya, or of the Sorata in Bolivia. All that lies at a greater depth below the level of the sea than the shafts or the basins of which I have spoken, the limits to which man's labors have penetrated, or than the depths to which the sea has in some few instances been sounded (Sir James Ross was unable to find bottom with 27,600 feet of line), is as much unknown to us as the interior of the other planets of our solar system. We only know the mass of the whole Earth and its mean density by comparing it with the open strata, which alone are accessible to us. In the interior of the Earth, where all knowledge of its chemical and mineralogical character fails, we are again limited to as pure conjecture, as in the remotest bodies that revolve round the Sun. We can determine nothing with certainty regarding the depth at which the geological strata must be supposed to be in state of softening or of liquid fusion, of the cavities occupied by elastic vapor, of the condition of fluids when heated under an enormous pressure, or of the law of the increase p 161 of density from the upper surface to the center of the Earth.

The consideration of the increase of heat with the increase of depth toward the interior of our planet, and of the reaction of the interior on the external crust, leads us to the long series of volcanic phenomena. These elastic forces are manifested in earthquakes, eruptions of gas, hot wells, mud volcanoes and lava currents from craters of eruption and even in producing alterations in the level of the sea.*

[footnote] * [See Daubeney 'On Volcanoes', 2d edit., 3848, p. 539, etc., on the so called 'mud volcanoes', and the reasons advanced in favor of adopting the term "salses" to designate these phenomena.] — Tr.

Large plains and variously indented continents are raised or sunk, lands are separated from seas, and the ocean itself, which is permeated by hot and cold currents, coagulates at both poles, converting water into dense masses of rock, which are either stratified and fixed, or broken up into floating banks. The boundaries of sea and land, of fluids and solids, are thus variously and frequently changed. Plains have undergone oscillatory movements, being alternately elevated and depressed. After the elevation of continents, mountain chains were raised upon long fissures, mostly parallel, and in that case, probably cotemporaneous; and salt lakes and inland seas, long inhabited by the same creatures, were forcibly separated, the fossil remains of shells and zoophytes still giving evidence of their original connection. Thus, in following phenomena in their mutual dependence, we are led from the consideration of the forces acting in the interior of the Earth to those which cause eruptions on its surface, and by the pressure of elastic vapors give rise to burning streams of lava that flow from open fissures.

The same powers that raised the chains of the Andes and the Hiimalaya to the regions of perpetual snow, have occasioned new compositions and new textures in the rocky masses, and have altered the strata which had been previously deposited from fluids impregnated with organic substances. We here trace the series of formations, divided and superposed according to their age, and depending upon the changes of configuration of the surface, the dynamic relations of upheaving forces, and the chemical action of vapors issuing from the fissures.

The form and distribution of continents, that is to say, of that solid portion of the Earth's surface which is suited to the luxurious development of vegetable life, are associated by intimate connection and reciprocal action with the encircling p 162 sea in which organic life is almost entirely limited to the animal world. The liquid element is again covered by the atmosphere, an aërial ocean in which the mountain chains and high plains of the dry land rise like shoals, occasioning a variety of currents and changes of temperature, collecting vapor from the region of clouds, and distributing life and motion by the action of the streams of water which flow from their declivities.

While the geography of plants and animals depends on these intricate relations of the distribution of sea and land, the configuration of the surface, and the direction of isothermal lines (or zones of equal mean annual heat), we find that the case is totally different when we consider the human race — the last and noblest subject in a physical description of the globe. The characteristic differences in races, and their relative numerical distribution over the Earth's surface, are conditions affected not by natural relations alone, but at the same time and specially, by the progress of civilization, and by moral and intellectual cultivation on which depends the political superiority that distinguishes national progress. Some few races, clinging, as it were, to the soil, are supplanted and ruined by the dangerous vicinity of others more civilized than themselves, until scarce a trace of their existence remains. Other races, again, not the strongest in numbers, traverse the liquid element, and thus become the first to acquire, although late, a geographical knowledge of at least the maritime lands of the whole surface of our globe, from pole to pole.

I have thus, before we enter on the individual characters of that portion of the delineation of nature which includes the sphere of telluric phenomena, shown generally in what manner the consideration of the form of the Earth and the incessant action of electro-magnetism and subterranean heat may enable us to embrace in one view the relations of horizontal expansion and elevation on the Earth's surface, the geognostic type of formations, the domain of the ocean (of the liquid portions of the Earth), the atmosphere with its meteorological processes, the geographical distribution of plants and animals, and, finally, the physical gradations of the human race, which is, exclusively and every where, susceptible of intellectual culture. This unity of contemplation presupposes a connection of phenomena according to their internal combination. A mere tabular arrangement of these facts would not fulfill the object I have proposed to myself, and would not satisfy that requirement for cosmical presentation awakened in me by the p 163 aspect of nature in my journeyings by sea and land, by the careful study of forms and forces, and by a vivid impression of the unity of nature in the midst of the most varied portions of the Earth. In the rapid advance of all branches of physical science, much that is deficient in this attempt will, perhaps, at no remote period, be corrected and rendered more perfect, for it belongs to the history of the development of knowledge that portions which have long stood isolated become gradually connected, and subject to higher laws. I only indicate the empirical path in which I and many others of similar pursuits with myself are advancing, full of expectation that, as Plato tells us Socrates once desired, "Nature may be interpreted by reason alone."*

[footnote] *Plato, 'Phaedo', p. 97. (Arist., 'Metaph.', p. 985.) compare Hegel, 'Philosophie der Geschichte', 1840, s. 16.

The delineation of the principal characteristics of telluric phenomena must begin with the form of our planet and its relations in space. Here too, we may say that it is not only the mineralogical character of rocks, whether they are crystalline, granular, or densely fossiliferous, but the geometrical form of the Earth itself, which indicates the mode of its origin, and is, in fact, its history. An elliptical spheroid of revolution gives evidence of having once been a soft or fluid mass. Thus the Earth's compression constitutes one of the most ancient geognostic events, as every attentive reader of the book of nature can easily discern; and an analogous fact is presented in the case of the Moon, the perpetual direction of whose axes toward the Earth, that is to say, the increased accumulation of matter on that half of the Moon which is turned toward us, determines the relations of the periods of rotation and revolution, and is probably contemporaneous with the earliest epoch in the formative history of this satellite. The mathematical figure of the Earth is that which it would have were its surface covered entirely by water in a state of rest; and it is this assumed form to which all geodesical measurements of degrees refer. This mathematical surface is different from that true physical surface which is affected by all the accidents and inequalities of the solid parts.*

[footnote] *Bessel, 'Allgemeine Betrachtungen uber Gradmessungen nach astronomisch-geodätischen Arbeiten', at the conclusion of Bessel and Baeyer, 'Gradmessung in Ostpreussen', s. 427. Regarding the accumulation of matter on the side of the Moon turned toward us (a subject noticed in an earlier part of the text), see Laplace, 'Expos. du Syst. du Monde', p. 308.

The whole figure of the Earth is determined when we know the amount of the p 164 compression at the poles and the equatorial diameter; in order, however, to obtain a perfect representation of its form it is necessary to have measurements in two directions, perpendicular to one another.

Eleven measurements of degrees (or determinations of the curvature of the
Earth's surface in different parts), of which nine only belong to the
present century, have made us acquainted with the size of our globe, which
Pliny names "a point in the immeasurable universe."*

[footnote] *Plin., ii., 68. Seneca, 'Nat. Quaest., Praef., c. ii. "El mundo espoco" (the Earth is small and narrow), writes Columbus from Jamaica to Queen Isabella on the 7th of July, 1503: not because he entertained the philosophic views of the aforesaid Romans, but because it appeared advantageous to him to maintain that the journey from Spain was not long, if, as he observes, "we seek the east from the west." Compare my 'Examen Crit. de l'Hist. de la Geogr. du 15 me Siecle', t.i., p. 83, and t. ii., p. 327, where I have shown that the opinion maintained by Delisle, Freret, and Gosselin, that the excessive differences in the statements regarding the Earth's circumference, found in the writings of the Greeks, are only apparent, and dependent on different values being attached to the stadia, was put forward as early as 1495 by Jaime Ferrer, in a proposition regarding the determination of the line of demarkation of the papal dominions.

If these measurements do not always accord in the curvatures of different meridians under the same degree of latitude, this very circumstance speaks in favor of the exactness of the instruments and the methods employed, and of the accuracy and the fidelity to nature of these partial results. The conclusion to be drawn from the increase of forces of attraction (in the direction from the equator to the poles) with respect to the figure of a planet is dependent on the distribution of density in its interior. Newton, from theoretical principles, and perhaps likewise prompted by Cassini's discovery, previously to 1666, of the compression of Jupiter,* determined, in his immortal work, 'Philosophiae Naturalis Principia', that the compression of the Earth, as a homogeneous mass, was 1/230th.

[footnote] *Brewster, 'Life of Sir Isaac Newton', 1831, p. 162. "The discovery of the spheroidal form of Jupiter by Cassini had probably directed the attention of Newton to the determination of its cause, and consequently, to the investigation of the true figure of the Earth." Although Cassini did not announce the amount of the compression of Jupiter (1/15th) till 1691 ('Anciens Memoires de l'Acad. des Sciences', t. ii., p. 108), yet we know from Lalande ('Astron.', 3me ed., t. iii., p. 335) that Moraldi possessed some printed sheets of a Latin work, "On the Spots of the Planets," commenced by Cassini, from which it was obvious that he was aware of the compression of Jupiter before the year 1666, and therefore at least twenty-one years before the publication of Newton's 'Principia'.

Actual mesurements, p 165 made by the aid of new and more perfect analysis, have, however, shown that the compression of the poles of the terrestrial spheroid, when the density of the strata is regarded as increasing toward the center, is very nearly 1/300th.

Three methods have been employed to investigate the curvature of the Earth's surface, viz., measurements of degrees, oscillations of the pendulum, and observations of the inequalities in the Moon's orbit. The first is a direct geometrical and astronomical method, while in the other two we determine from accurately observed movements the amount of the forces which occasion those movements, and from these forces we arrive at the cause from whence they have originated, viz., the compression of our terrestrial spheroid. In this part of my delineation of nature, contrary to my usual practice, I have instanced methods because their accuracy affords a striking illustration of the intimate connection existing among the forms and forces of natural phenomena, and also because their application has given occasion to improvements in the exactness of instruments (as those employed in the measurements of space) in optical and chronological observations; to greater perfection in the fundamental branches of astronomy and mechanics in respect to lunar motion and to the resistance experienced by the oscillations of the pendulum; and to the discovery of new and hitherto untrodden paths of analysis. With the exception of the investigations of the parallax of stars, which led to the discovery of aberration and nutation, the history of science presents no problem in which the object attained — the knowledge of the compression and of the irregular form of our planet — is so far exceeded in importance by the incidental gain which has accrued, through a long and weary course of investigation, in the general furtherance and improvement of the mathematical and astronomical sciences. The comparison of eleven measurements of degrees (in which are included three extra-European, namely, the old Peruvian and two East Indian) gives, according to the most strictly theoretical requirements allowed for by Bessel,* a compression p 166 of 1/299th.

[footnote] *According to Bessel's examination of ten measurements of degrees, in which the error discovered by Poissant in the calculation of the French measurements is taken into consideration (Schumacher, 'Astron. Nachr.', 1841, No. 438, s. 116), the semi-axis major of the elliptical spheroid of revolution to which the irregular figure of the Earth most closely approximates is 3,272,077.14 toises, or 20,924,774 feet; the semi-axis minor, 3,261,159,83 toises, or 20,854,821 feet; and the amount of compression or eccentricity 1/299.152d; the length of a mean degree of the meridian, 57,013.109 toises, or 364,596 feet, with an error of + 2.8403 toises, or 18.16 feet, whence the length of a geographical mile is 3807.23 toises, or 6086.7 feet. Previous combinations of measurements of degrees varied between 1/302d and 1/297th; thus Walbeck ('De Forma of Magnitudine telluris in demensis arcubus Meridiani definiendis', 1819) gives 1/30278th: Ed. Schmidt ('Lehrbuch der Mathem. und Phys. Geographie', 1829, s. 5) gives 1/20742d, as the mean of seven measures. Respecting the influence of great differences of longitude on the polar compression, see 'Bibliotheque Universelle', t. xxxiii., p. 181, and t. xxxv., p. 50: likewise 'Connaissance des Tems', 1829, p. 290. From the lunar inequalities alone, Laplace ('Exposition du Syst. du Monde', p. 229) found it, by the older tables of Burg, to be 1/3245th; and subsequently, from the lunar observations of Burckhardt and Bouvard, he fixed it at 1/299.1th ('Mecanique Celeste', t. v., p. 13 and 43).

In accordance with this, the polar radius is 10,938 toises (69,944 feet), or about 11 1/2 miles, shorter than the equatorial radius of our terrestrial spheroid. The excess at the equator in consequence of the curvature of the upper surface of the globe amounts, consequently, in the direction of gravitation, to somewhat more than 4 3/7th times the height of Mont Blanc, or only 2 1/2 times the probable height of the summit of the Chawalagiri, in the Himalaya chain. The lunar inequalities (perturbation in the moon's latitude and longitude) give according to the last investigations of Laplace, almost the same result for the ellipticity as the measurements of degrees, viz., 1/299th. The results yielded by the oscillation of the pendulum give, on the whole, a much greater amount of compression, viz., 1/288th.*

[footnote] *The oscillations of the pendulum give 1/288.7th as the general result of Sabine's great expedition (1822 and 1823, from the equator to 80 degrees north latitude); according to Freycinet, 1/286.2d, exclusive of the experiments instituted at the Isle of France, Guam, and Mowi (Mawi); according to Forster, 1/289.5th; according to Duperrey, 1/266.4th; and according to Lutke ('Partie Nautique', 1836, p. 232), 1/270th, calculated from eleven stations. On the other hand, Mathieu ('Connais. des Temps', 1816, p. 330) fixed the amount at 1/298.2d, from observations made between Formentera and Dunkirk; and Biot, at 1/304th, from observations between Formentera and the island of Ust. Compare Baily, 'Report on Pendulum Experiments', in the 'Memoirs of the Royal Astronomical Society', vol. vii., p. 96; also Borenius, in the 'Bulletin de l'Acad. de St. Petersbourg', 1843, t. i., p. 25. The first proposal to apply the length of the pendulum as a standard of measure, and to establish the third part of the seconds pendulum (then supposed to be every where of equal length) as a 'pes horarius', or general measure, that might be recovered at any age and by all nations, is to be found in Huygens's 'Horologium Oscillatorium', 1673, Prop. 25. A similar wish was afterward publicly expressed, in 1742, on a monument erected at the equator by Bouguer, La Condamine, and Godin. On the beautiful marble tablet which exists, as yet uninjured, in the old Jesuits' College at Quito, I have myself read the inscription, 'Penduli simplicis aequinoctialis unius minuti secundi archetypus, mensurae naturalis exemplar, utinam universalis!' From an observation made by La Condamine, in his 'Journal du Voyage a l'Equateur', 1751, p. 163, regarding parts of the inscription that were not filled up, and a slight difference between Bonguer and himself respecting the numbers, I was led to expect that I should find considerable discrepancies between the marble tablet and the inscription as it had been described in Paris; but, after a careful comparison, I merely found two "ex arca graduum plusquam trium," and the date of 1745 instead of 1742. The latter circumstance is singular, because La Condamine returned to Europe in November, 1744, Bouguer in June of the same year, and Godin had left South America in July, 1744. The most necessary and useful amendment to the numbers on this inscription would have been the astronomical longitude of Quito. (Humboldt, 'Recueil d'Observ. Astron.', t. ii., p. 319-354.) Nouet's latitudes, engraved on Egyptian monuments, offer a more recent example of the danger presented by the grave perpetuation of false or careless results.

Galileo, who first observed when a boy (having, probably, suffered his thoughts to wander from the service) that the height of the vaulted roof of a church might be measured by the time of the vibration of the chandeliers suspended at different altitudes, could hardly have anticipated that the pendulum would one day be carried from pole to pole, in order to determine the form of the Earth, or, rather, that the unequal density of the strata of the Earth affects the length of the seconds pendulum by means of intricate forces of local attraction, which are, however, almost regular in large tracts of land. These geognostic relations of an instrument intended for the measurement of time — this property of the pendulum, by which, like a sounding line, it searches unknown depths, and reveals in volcanic islands,* or in the declivity of elevated continental mountain chains,** dense masses of basalt and melaphyre instead of cavities, render it difficult, notwithstanding the admirable simplicity of the method, to arrive at any great result regarding the figure of the Earth from observation of the oscillations of the pendulum.

[footnote] *Respecting the augmented intensity of the attraction of gravitation in volcanic islands (St. Helena, Ualan, Fernando de Noronha, Isle of France, Guam, Mowe, and Galapagos), Rawak (Lutke, p. 240) being an exception, probably in consequence of its proximity to the highland of New Guinea, see Mathieu, in Delambre, 'Hist. de l'Astronomie, au 18me Siecle', p. 701.

[footnote] **Numerous observations also show great irregularities in the length of the pendulum in the midst of continents, and which are ascribed to local attractions. (Delambre, 'Mesure de la Meridienne', t. iii., p. 548; Biot, in the 'Mem. de l'Academie des Sciences', t. viii., 1829, p. 18 and 23.) In passing over the South of France and Lombardy from west to east, we find the minimum intensity of gravitation at Bordeaux; from thence it increases rapidly as we advance eastward, through Figeac, Clermont-Ferrand, Milan, and Padua; and in the last town we find that the intensity has attained its maximum. The influence of the southern declivities of the Alps is not merely t on the general size of their mass, but (much more), in the opinion of Elie de Beaumont ('Rech. sur les Revol. de la Surface du Globe', 1830, p. 729), on the rocks of melaphyre and serpentine, which have elevated the chain. On the declivity of Ararat, which with Caucasus may be said to lie in the center of gravity of the old continent formed by Europe, Asia, and Africa, the very exact pendulum experiments of Fedorow give indications, not of subterranean cavities, but of dense volcanic masses. (Parrot, 'Reise zum Ararat', bd. ii., s. 143.) In the geodesic operations of Carlini and Plana, in Lombardy, differences ranging from 20" to 47".8 have been found between direct observations of latitude and the results of these operations. (See the instances of Andrate and Mondovi, and those of Milan and Padua, in the 'Operations Geodes. et Astron. pour la Mesure d'un Arc du Parallele Moyen', t. ii., p. 347; 'Effemeridi Astron. di Milano', 1842, p. 57.) The latitude of Milan, deduced from that of Berne, according to the , is 45ºdegrees 27' 52", while, according to direct astronomical observations, it is 45 degrees 27' 35". As the perturbations extend in the plain of Lombardy to Parma, which is far south of the Po (Plana, 'Operat. Geod.', t. ii., p. 847), it is probable that there are deflecting causes 'concealed beneath the soil of the plain itself'. Struve has made similar experiments [with corresponding results] in the most level parts of eastern Europe. (Schumacher, 'Astron. Nachrichten', 1830, No. 164, s. 399.) Regarding the influence of dense masses supposed to lie at a small depth, equal to the mean height of the Alps, see the analytical expressions given by Hossard and Rozet, in the 'Comptes Rendus', t. xviii., 1844, p. 292, and compare them with Poisson, 'Traite de Mecanique' (2me ed., t. i., p. 482. The earliest observations on the influence which different kinds of rocks exercise on the vibration of the pendulum are those of Thomas Young, in the 'Philos. Transactions' for 1819, p. 70-96. In drawing conclusions regarding the Earth's curvature from the length of the pendulum, we ought not to overlook the possibility that its crust may have undergone a process of hardening previously to metallic and dense basaltic masses having penetrated from great depths, through open clefts, and approached near the surface.

In the astronomical part of the determination of degrees of latitude, mountain chains, or the denser strata of the Earth, likewise exercise, although in a less degree, an unfavorable influence on the measurement.

As the form of the Earth exerts a powerful influence on the motions of other cosmical bodies, and especially on that of its own neighboring satellite, a more perfect knowledge of the motion of the latter will enable us reciprocally to draw an inference regarding the figure of the Earth. Thus, as Laplace ably remarks,* "An astronomer, without leaving his observatory, may, by a comparison of lunar theory with true observations, not only be enabled to determine the form and size of the Earth, but also its distance from the Sun and Moon — results that otherwise could only be arrived at by long and arduous expeditions to the most remote parts of both hemispheres."

[footnote] *Laplace, 'Expos. du Syst. du Monde', p. 231.

p 169 The compression which may be inferred from lunar inequalities affords an advantage not yielded by individual measurements of degrees or experiments with the pendulum, since it gives a mean amount which is referable to the whole planet. The comparison of the Earth's compression with the velocity of rotation shows, further, the increase of density from the strata from the surface toward the center — an increase which a comparison of the ratios of the axes of Jupiter and Saturn with their times of rotation likewise shows to exist in these two large planets. Thus the knowledge of the external form of planetary bodies leads us to draw conclusions regarding their internal character.

The northern and southern hemispheres appear to present nearly the same curvature under equal degrees of latitude, but, as has already been observed, pendulum experiments and measurements of degrees yield such different results for individual portions of the Earth's surface that no regular figure can be given which would reconcile all the results hitherto obtained by this method. the true figure of the Earth is to a regular figure as the uneven surfaces of water in motion are on the even surface of water at rest.

When the Earth had been measured, it still had to be weighed. The oscillations of the pendulum* and the plummet have here likewise served to determine the mean density of the Earth, either in connection with astronomical and geodetic operations, with the view of finding the deflection of the plummet from a vertical line in the vicinity of a mountain, or by a comparison of the length of the pendulum in a plain and on the summit of an elevation, or, finally, by the employment of a torsion balance, which may be considered as a horizontally vibrating pendulum for the measurement of the relative density of neighbouring strata.

[footnote] *La Caille's pendulum measurements at the Cape of Good Hope, which have been calculated with much care by Mathieu (Delambre, 'Hist. de l'Astron. au 18me Siecle', p. 479), give a compression of 1/284.4th; but, from several comparisons of observations made in equal latitudes in the two hemispheres (New Holland and the Malouines (Falkland Islands), compared with Barcelona, New York, and Dunkirk), there is as yet no reason for supposing that the mean compression of the southern hemisphere is greater than that of the northern. (Biot, in the 'Mem. de l'Acad. des Sciences', t. viii., 1829, p. 39-41.)

Of these three methods* the p 170 last is the most certain, since it is independent of the difficult determination of the density of the mineral masses of which the spherical segment of the mountain consists near which the observations are made.

[footnote] *The three methods of observation give the following results: (1.) by the deflection of the plumb-line in the proximity of the Shehallien Mountain (Gaelic, Thichallin) in Perthshire, r.713, as determined by Maskelyne, Hutton, and Playfair (1774-1776 and 1810), according to a method that had been proposed by Newton; (2.) by pendulum vibrations on mountains, 4.837 (Carlini's observations on Mount Cenis compared with Biot's observations at Bordeaux, 'Effemer. Astron. di Milano', 1824, p. 184); (3.) by the torsion balance used by Cavendish, with an apparatus originally devised by Mitchell, 5.48 (according to Hutton's revision of the calculation, 5.32, and according to that of Eduard Schmidt, 5.52; 'Lehrbuch der Math. Geographie', bd. i., s. 487); by the torsion balance, according to Reich, 5.44. In the calculation of these experiments of Professor Reich, which have been made with masterly accuracy, the original mean result was 5.43 (with a probable error of only 0.0233), a result which, being increased by the quantity by which the Earth's centrifugal force diminishes the force of gravity for the latitude of Freiberg (50 degrees 55'), becomes changed to 5.44. The employment of cast iron instead of lead has not presented any sensible difference, or none exceeding the limits of errors of observation, hence disclosing no traces of magnetic influences. (Reich, 'Vrsuche uber die mittlere Dichtigheit der Erde', 1838, s. 60, 62, and 66.) By the assumption of too slight a degree of ellipticity of the Earth, and by the uncertainty of the estimations regarding the density of rocks on its surface, the mean density of the Earth, as deduced from experiments on and near mountains, was found about one sixth smaller than it really is, namely, 4.761 (Laplace, 'Mecan. Celeste', t. v., p. 46), or 4.785. (Eduard Schmidt, 'Lehrb. der Math. Geogr.', bd. i., 387 und 418.) On Halley's hypothesis of the Earth being a hollow sphere (noticed in page 171), which was the germ of Franklin's ideas concerning earthquakes, see 'Philos. Trans.' for the year 1693, vol. xvii., p. 563 ('On the Structure of the Internal Parts of the Earth, and the concave habited 'Arch of the Shell'). Halley regarded it as more worthy of the Creator "that the Earth, like a house of several stories, should be inhabited both without and within. For light in the hollow sphere (p. 576) provision might in some manner be contrived."

According to the most recent experiments of Reich, the result obtained is 5.44; that is to say, the mean density of the whole Earth is 5.44 times greater than tht of pure water. As according to the nature of the mineralogical strata constituting the dry continental part of the Earth's surface, the mean density of this portion scarcely amounts to 2.7, and the density of the dry and liquid surface conjointly to scarcely 1.6, it follows that the elliptical unequally compressed layers of the interior must greatly increase in density toward the center, either through pressure or owing to the heterogeneous nature of the substances. Here again we see that the vertical, as well as the horizontally vibrating pendulum, may justly be termed a geognostical instrument.

The results obtained by the employment of an instrument of this kind have led celebrated physicists, according to the difference of the hypothesis from which they started, to adopt p 171 entirely opposite views regarding the nature of the interior of the globe. It has been computed at what depths liquid or even gaseous substances would, from the pressure of their own superimposed strata, attain a density exceeding that of platinum or even iridium; and in order that the compression which has been detrmined within such narrow limits might be brought into harmony with the assumption of simple and infinitely compressible matter, Leslie has ingeniously conceived the nucleus of the world to be a hollow sphere, filled with an assumed "imponderable matter, having an enormous force of expansion." These venturesome and arbitrary conjectures have given rise, in wholly unscientific circles, to still more fantastic notions. The hollow sphere has by degrees been peopled with plants and animals, and two small subterranean revolving planets — Pluto and Proserpine — were imaginatively supposed to shed over it their mild light; as, however, it was further imagined that an ever-uniform temperature reigned in these internal regions, the air, which was made self-luminous by compression, might well render the planets of this lower world unnecessary. Near the north pole, at 80 degrees latitude, whence the polar light emanates, was an enormous opening, through which a descent might be made into the hollow sphere, and Sir Humphrey Davy and myself were even publicly and frequently invited by Captain Symmes to enter upon this subterranean expedition: so powerful is the morbid inclination of men to fill unknown spaces with shapes of wonder, totally unmindful of the counter evidence furnished by well-attested facts and universally acknowledged natural laws. Even the celebrated Halley, at the end of the seventeenth century, hollowed out the Earth in his magnetic speculations. Men were invited to believe that a subterranean freely-rotating nucleus occasions by its position the diurnal and annual changes of magnetic declination. It has thus been attempted in our own day, with tedious solemnity, to clothe in a scientific garb the quaintly-devised fiction of the humorous Holbert.*

[footnote] *[The work referred to, one of the wittiest productions of the learned Norwegian satirist and dramatist Holberg, was written in Latin, and first appeared under the following title: 'Nicolai Klimii iter subterraneum novam telluris theoriam ac historiam quintae monarchi Nicolai Klimii iter subterraneum novam telluris theoriam ac historiam quintae monarchi ad huc nobis incognitae exhibens e bibliotheca b. Abelini. Hafniae et Lipsiae sunt. Jac. Preuss', 1741. An admirable Danish translation of this learned but severe satire on the institutions, morals, and manners of the inhabitants of the upper Earth, appeared at Copenhagen in 1789, and was entitled 'Niels Klim's underjordiske reise ocd Ludwig Holberg, oversal after den Latinske original of Jens Baggesen'. Holberg, who studied for a time at Oxford, was born at Bergen in 1685, and died in 1754 as Rector of the University of Copenhagen.] — Tr.

p 172 The figure of the Earth and the amount of solidification (density) which it has acquired are intimately connected with the forces by which it is animated, in so far, at least, as they have been excited or awakened from without, through its planetry position with reference to a luminous central body. Compression, when considered as a consequence of centrifugal force acting on a rotating mass, explains the earlier condition of fluidity of our planet. During the solidification of this fluid, which is commonly conjectured to have been gaseous and primordially heated to a very high temperature, an enormous quantity of latent heat must have been liberated. If the process of solidification began as Fourier conjectures, by radiation from the cooling surface exposed to the atmosphere, the particles near the center would have continued fluid and hot. As, after long emanation of heat from the center toward the exterior, a stable condition of the temperature of the Earth would at length be established, it has been assumed that with increasing depth the subterranean heat likewise uninterruptedly increases. The heat of the water which flows from deep borings (Artesian wells), direct experiments regarding the temperature of rocks in mines, but, above all, the volcanic activity of the Earth, shown by the flow of molten masses from open fissures, afford unquestionable evidence of this increase for very considerable depths from the upper strata. According to conclusions based certainly upon mere analogies, this increase is probably much greater toward the center.

That which has been learned by an ingenious analytic calculation, expressly perfected for this class of investigations,* p 173 regarding the motion of heat in homogeneous metallic spheroids, must be applied with much caution to the actual character of our planet, considering our present imperfect knowledge of the substances of which the Earth is composed, the difference in the capacity of heat and in the conducting power of different superimposed masses, and the chemical changes experienced by solid and liquid masses from any enormous compression.

[footnote] *Here we must notice the admirable analytical labors of Fourier, Biot, Laplace, Poisson, Duhamel, and Lame. In his 'Theorie Mathematique de la Chaleur', 1835, p. 3, 428-430, 436, and 521-524 (see, also, De la Rive's abstract in the 'Bibliotheque Universelle de Geneve', Poisson has developed an hypothesis totally different from Fourier's view ('Theorie Analytique de la Chaleur'.) He denies the present fluid state of the Earth's center; he believes that "in cooling by radiation to the medium surrounding the Earth, the parts which were first solidified sunk, and that by a double descending and ascending current, the great inequality was lessened which would have taken place in a solid body cooling from the surface." It seems more probable to this great geometer that the solidification began in the parts lying nearest to the center: "the phenomenon of the increase of heat with the depth does not extend to the whole mass of the Earth, and is merely a consequence of the motion of our planetary system in space, of which some parts are of a very different temperature from others, in consequence of stellar heat (chaleur stellaire)." Thus, according to Poisson, the warmth of the water of our Artesian wells is merely that which has penetrated into the Earth from without; and the Earth itself "might be regarded as in the same circumstances as a mass of rock conveyed from the equator to the pole in so short a time as not to have entirely cooled. The increase of temperature in such a block would not extend to the central strata." The physical doubts which have reasonably been entertained against this extraordinary cosmical view (which attributes to the regions of space that which probably is more dependent on the first transition of matter condensing from the gaseo-fluid into the solid state) will be found collected in Poggendorf's 'Annalen', bd. xxxix., s 93-100.

It is with the greatest difficulty that our powers of comprehension can conceive the boundary line which divides the fluid mass of the interior from the hardened mineral masses of the external surface, or the gradual increase of the solid strata, and the condition of semi-fluidity of the earthy substances, these being conditions to which known laws of hydraulics can only apply under considerable modifications. The Sun and Moon, which cause the sea to ebb and flow, most probably also affect these subterranean depths. We may suppose that the periodic elevations and depressions of the molten mass under the already solidified strata must have caused inequalities in the vaulted surface from the force of pressure. The amount and action of such oscillations must, however, be small; and if the relative position of the attracting cosmical bodies may here also excite "spring tides," it is certainly not to these, but to more powerful internal forces, that we must ascribe the movements that shake the Earth's surface. There are groups of phenomena to whose existence it is necessary to draw attention, in order to indicate the universality of the influence of the attraction of the Sun and Moon on the external and internal conditions of the Earth, however little we may be able to determine the quantity of this influence.

According to tolerably accordant experiments in Artesian wells, it has been shown that the heat increases on an average about 1 degree for every 54.5 feet. If this increase can be reduced p 174 to arithmetical relations, it will follow, as I have already observed,* that a stratum of granite would be in a state of fusion at a depth of nearly twenty-one geographical miles, or between four and five times the elevation of the highest summit of the Hinalaya.

[footnote] *See the Introduction. This increase of temperature has been found in the Puits de Grenelle, at Paris, at 58.3 feet; in the boring at the new salt-works at Minden, almost 53.6; at Pregny, near Geneva, according to Auguste de la Rive and Marcet, notwithstanding that the mouth of the boring is 1609 feet above the level of the sea, it is also 53.6 feet. This coincidence between the results of a method first proposed by Arago in the year 1821 ('Annuaire du Bureau des Longitudes', 1835, p. 234), for three different mines, of the absolute depths of 1794, 2231, and 725 feet respectively, is remarkable. The two points on the Earth, lying at a small vertical distance from each other, whose annual mean temperatures are most accurately known, are probably at the spot on which the Paris Observatory stands, and the Caves de l'Observatoire beneath it; the mean temperature of the former is 51.5ºdegrees, and of the latter 53.3ºdegrees, the difference being 1.8ºdegrees for 92 feet, or 1 degree for 51.77 feet. (Poisson, 'Theorie Math. de la Chaleur', p. 415 and 462.) In the course of the last seventeen years, from causes not yet perfectly understood, but probably not connected with the actual temperature of the caves, the thermometer standing there has risen very nearly 0.4 degrees. Although in Artesian wells there are sometimes slight errors from the lateral permeation of water, these errors are less injurious to the accuracy of conclusions than those resulting from currents of cold air, which are almost always present in mines. The general result of Reich's great work on the temperature of the mines in the Saxony mining districts gives a somewhat slower increase of the terrestrial heat, or 1 degree to 76.3 feet. (Reich, 'Beob. uber die Temperatur des Gesteins in verschielen en Tiefen', 1834, s. 134.) Phillips, however, found (Pogg., 'Annalen', bd. xxxiv., s. 191), in a shaft of the coal-mine of Monk-wearmouth, near Newcastle, in which, as I have already remarked, excavations are going on at a depth of about 1500 feet below the level of the sea, an increase of 1 degree to 59.06 feet, a result almost identical with that found by Arago in the Puits de Grenell.

We must distinguish in our globe three different modes for the transmission of heat. The first is periodic, and affects the temperature of the terrestrial strata according as the heat penetrates from above downward or from below upward, being influenced by the different positions of the Sun and the seasons of the year. The second is likewise an effect of the Sun, although extremely slow: a portion of the heat that has penetrated into the equatorial regions moves in the interior of the globe toward the poles, where it escapes into the atmosphere and the remoter regions of space. The third mode of transmission is the slowest of all, and is derived from the secular cooling of the globe, and from the small portion of the primitive heat which is still being disengaged from the surface. p 175 This loss experienced by the central heat must have been very considerable in the earliest epochs of the Earth's revolutions, but within historical periods it has hardly been appreciable by our instruments. The surface of the Earth is therefore situated between the glowing heat of the inferior strata and the universal regions of space, whose temperature is probably below the freezing-point of mercury.

The periodic changes of temperature which have been occasioned on the Earth's surface by the Sun's position and by meteorological processes, are continued in its interior, although to a very inconsiderable depth. The slow conducting power of the ground diminishes this loss of heat in the winter, and is very favorable to deep-rooted trees. Points that lie at very different depths on the same vertical line attain the maximum and minimum of the imparted temperature at very different periods of time. The further they are removed from the surface, the smaller is this difference between the extremes. In the latitudes of our temperate zone (between 48 degrees and 52 degrees), the stratum of invariable temperature is at a depth of from 59 to 64 feet, and at half that depth the oscillations of the thermometer, from the influence of the seasons, scarcely amount to half a degree. In tropical climates this invariable stratum is only one foot below the surface, and this fact has been ingeniously made use of by Boussingault to obtain a convenient, and as he believes, certain determination of the mean temperature of the air of different places.*

[footnote] *Boussingault, 'Sur la Profondeus a laquelle se trouve la Couche de Temperature invariable, entre les Tropiques', in the 'Annales de Chimie et de Physique', t. liii., 1833, p. 225-247.

This mean temperature of the air at a fixed point, or at a group of contiguous points on the surface, is to a certain degree the fundamental element of the climate and agricultural relations of a district; but the mean temperature of the whole surface is very different from that of the globe itself. The questions so often agitated, whether the mean temperature has experienced any considerable differences in the course of centuries, whether the climate of a country has deteriorated, and whether the winters have not become milder and the summers cooler, can only be answered by means of the thermometer; this instrument has, however, scarcely been invented more than two centuries and a half, and its scientific application hardly dates back 120 years. The nature and novelty of the means interpose, therefore, very narrow limits to our investigation regarding the temperature p 176 of the air. It is quite otherwise, however, with the solution of the great problem of the internal heat of the whole Earth. As we may judge of uniformity of temperature from the unaltered time of vibration of a pendulum, so we may also learn, from the unaltered rotatory velocity of the Earth, the amount of stability in the mean temperature of our globe. This insight into the relations between the 'length of the day' and the 'heat of the Earth' is the result of one of the most brilliant applications of the knowledge we had long possessed of the planet. The rotatory velocity of the Earth depends on its volume; and since, by the gradual cooling of the mass by radiation, the axis of rotation would become shorter, the rotatory velocity would necessarily increase, and the length of the day diminish, with a decrease of the temperature. From the comparison of the secular inequalities in the motions of the Moon with the eclipses observed in ancient times, it follows that, since the time of Hipparchus, that is, for full 2000 years, the length of the day has certainly not diminished by the hundredth part of a second. The decrease of the mean heat of the globe during a period of 2000 years has not, therefore, taking the extremest limits, diminished as much as 1/306th of a degree of Fahrenheit.*

[footnote] *Laplace, 'Exp. du Syst. du Monde', p. 229 and 263; 'Mecanique Celeste', t. v., p. 18 and 72. It should be remarked that the fraction 1/306th of a degree of Fahrenheit of the mercurial thermometer, given in the text as the limit of the stability of the Earth's temperature since the days of Hipparchus, rests on the assumption that the dilation of the substances of which the Earth is composed is equal to that of glass, that is to say, 1/18,000th for 1 degree. Regarding this hypothesis, see Arago in the 'Annuaire' for 1834, p. 177-190.

This invariability of form presupposes also a great invariability in the distribution of relations of density in the interior of the globe. The translatory movements, which occasion the eruptions of our present volcanoes and of ferruginous lava, and the filling up of previously empty fissures and cavities with dense masses of stone, are consequently only to be regarded as slight superficial phenomena affecting merely one portion of the Earth's crust, which, from their smallness when compared to the Earth's radius, become wholly insignificant.

I have described the internal heat of our planet, both with reference to its cause and distribution, almost solely from the results of Fourier's admirable investigations. Poisson doubts the fact of the uninterrupted increase of the Earth's heat p 177 from the surface to the center, and is of opinion that all heat has penetrated from without inward, and that the temperature of the globe depends upon the very high or very low temperature of the regions of space through which the solar temperature of the regions of space, through which the solar system has moved. This hypothesis, imagined by one of the most acute mathematicians of our time, has not satisfied physicists or geologists, or scarcely indeed any one besides its author. But, whatever may be the cause of the internal heat of our planet, and of its limited or unlimited increase in deep strata, it leads us, in this general sketch of nature, through the intimate connection of all primitive phenomena of matter, and through the common bond by which molecular forces are united, into the mysterious domain of magnetism. Changes of temperature call forth magnetic and electric currents. Terrestrial magnetism, whose main character, expressed in the three-fold manifestation of its forces, is incessant periodic variability, is ascribed either to the heated mass of the Earth itself,* or to those galvanic currents which we consider as electricity in motion, that is, electricity moving in a closed circuit.**

[footnote] *William Gilbert, of Colchester, whom Galileo pronounced "great to a degree that might be envied," said "magnus magnes ipse est globus terrestris." He ridicules the magnetic mountains of Frascatori, the great contemporary of Columbus, as being magnetic poles: "rejicienda est vulgaris opinio de montibus magneticis, aut rupe aliqua magnetica, aut polo phantastico a polo mundi distante." He assumes the declination of the magnetic needle at any give point on the surface of the Earth to be invariable (variatio uniuscujusque loci constans est), and refers the curvatures of the isogonic lines to the configuration of continents and the relative positions of sea basins, which possess a weaker magnetic force than the solid masses rising above the ocean. (Gilbert, 'de Magnete', ed. 1633, p. 42, 98, 152 and 155.)

[footnote] ** Gauss, 'Allgemcine Theorie des Erdmagnetismus', in the 'Resultate aux den Beob. des Magnet. Vereins', 1838, s. 41, p. 56.

The mysterious course of the magnetic needle is equally affected by time and space, by the sun's course, and by changes of place on the Earth's surface. Between the tropics, the hour of the day may be known by the direction of the needle as well as by the oscillations of the barometer. It is affected instantly, but only transiently, by the distant northern light as it shoots from the pole, flashing in beams of colored light across the heavens. When the uniform horary motion of the needle is disturbed by a magnetic storm, the perturbation manifests itself 'simultaneously', in the strictest sense of the word, over hundreds and thousands of miles of sea and land, or propagates itself by degrees, in short intervals of time, in p 178 every direction over the Earth's surface.*

[footnote] *There are also perturbations which are of a local character, and do not extend themselves far, and are probably less deep-seated. Some years ago I described a rare instance of this kind, in which an extraordinary disturbance was felt in the mines at Freiberg, but was not perceptible at Berlin. ('Lettre de M. de Humboldt a Son Altesse Royale le Duc de Sussex sur les moyens propres a perfectionner la Connaissance du Magnetisme Terrestre', in Becquerel's 'Traite Experimental de l'Electricite' t. vii., p. 442.) Magnetic storms which were simultaneously felt from Sicily to Upsala, did not extend from Upsala to Alten. (Gauss and Weber, 'Resultate des Magnet. Vereins', 1839, 128; Lloyd, in the 'Comptes Rendus de l'Acad. des Sciences', t. xii., 1843, Sem. ii., p. 725 and 827.) Among the numerous examples that have been recently observed, of perturbations occurring simultaneously and extending over wide portions of the Earth's surface, and which are collected in Sabine's important work ('Observ. on Days of unusual Magnetic Disturbance', 1843), one of the most remarkable is that of the 25th of September, 1841, which was observed at Toronto in Canada, at the Cape of Good Hope, at Prague, and partially in Van Diemen's Land. The English Sunday, on which it is deemed sinful, after midnight on Saturday, to register an observation, and to follow out the great phenomena of creation in their perfect development, interrupted the observations in Van Diemen's Land, where in consequence of the difference of the longitude, the magnetic storm fell on the Sunday. ('Observ.', p. xiv., 78, 85, and 87.)

In the former case, the simultaneous manifestation of the storm may serve, within certain limitations, like Jupiter's satellites, fire-signals, and well-observed falls of shooting stars, for the geographical determination of degrees of longitude. We here recognize with astonishment that the perturbations of two small magnetic needles, even if suspended at great depths below the surface, can measure the distances apart at which they are placed, teaching us, for instance, how far Kasan is situated east of Gottingen or of the banks of the Seine. There are also districts in the earth where the mariner, who has been enveloped for many days in mist, without seeing either the sun or stars, and deprived of all means of determining the time, may know with certainty, from the variations in the inclination of the magnetic needle, whether he is at the north or the south of the port he is desirous of entering.*

[footnote] *I have described, in Lametherie's 'Journal de Physique', 1804, t. lix., p. 449, the application (alluded to in the text) of the magnetic inclination to the determination of latitude along a coast running north and south, and which, like that of Chili and Peru, is for a part of the year enveloped in mist ('garua'). In the locality I have just mentioned, this application is of the greater importance, because, in consequence of the strong current running northward as far as to Cape Parena, navigators incur a great loss of time if they approach the coast to the north of the haven they are seeking. In the South Sea, from Callao de Lima harbor to Truxillo, which differ from each other in latitude by 3 degrees 57' I have observed a variation of the magnetic inclination amounting to 9 degrees (centesimal division); and from Callao to Guayaquil, which differ in latitude by 9 degrees 50', a variation of 23.5 degrees. (See my 'Relat. Hist.', t. iii., p. 622.) At Guarmey (10 degrees 4' south lat.), Huaura (11 degrees 3' south lat.), and Chancay (11 degrees 4' south lat.), Huaura (11 degrees 3' south lat.), and Chancay (11 degrees 32' south lat.), the inclinations are 6.80 degrees, 9 degrees, and 10.35 degrees of the centesimal division. The determination of position by means of the magnetic inclination has this remarkable feature connected with it, that where the ship's course cuts the isoclinalline almost perpendicularly, it is the only one that is independent of all determination of time, and consequently, of observations of the sun or stars. It is only lately that I discovered, for the first time, that as early as at the close of the sixteenth century, and consequently hardly twenty years after Robert Norman had invented the inclinatorium, William Gilbert, in his great work, 'De Magnete', proposed to determine the latitude by the inclination of the magnetic needle. Gilbert ('Physiologia Nova de Magnete', lib. v., cap. 8, p. 200) commends the method as applicable "aëre caliginoso." Edward Wright, in the introduction which he added to his master's great work, describes this proposal as "worth much gold." As he fell into the same error with Gilbert, of presuming that the isoclinal lines coincided with the geographical parallel circles, and that the magnetic and geographical equators were identical, he did not perceive that the proposed method had only a local and very limited application.

p 179 When the needle, by its sudden disturbance in its horary course, indicates the presence of a magnetic storm, we are still unfortunately ignorant whether the seat of the disturbing cause is to be sought in the Earth itself or in the upper regions of the atmosphere. If we regard the Earth as a true magnet, we are obliged, according to the views entertained by Friedrich Gauss (the acute propounder of a generaltheory of terrestrial magnetism), to ascribe to every portion of the globe measuring one eighth of a cubic meter (or 3 7/10ths of a French cubic foot) in volume, an average amount of magnetism equal to that contained in a magnetic rod of 1 lb. weight.*

[footnote[ *Gauss and Weber, 'Resultate des Magnet. Vereins', 1838, 31, s. 146.

If iron and nickel, and probably, also, cobalt (but not chrome, as has long been believed),* are the only substances which become permanently magnetic, and retain polarity from a certain coerceive force, the phenomena of Arago's magnetism of rotation and of Faraday's induced currents show, on the other hand, that all telluric substances may possibly be made transitorily magnetic.

According to Faraday ('London and Edinburgh Philosophical Magazine', 1836, vol. viii., p. 178), pure cobalt is totally devoid of magnetic power. I know, however, that other celebrated chemists (Heinrich Rose and Wohler) do not admit this as absolutely certain. If out of two carefully-purified masses of cobalt totally free from nickel, one appears altogether non-magnetic (in a state of equilibrium), I think it probable that the other owes its magnetic property to a want of purity; and this opinion coincides with Faraday's view.

According to the experiments of the p 180 first-mentioned of these great physicists, water, ice, glass, and carbon affect the vibrations of the needle entirely in the same manner as mercury in the rotation experiments.*

[footnote] *Arago, in the 'Annales de Chimie', t. xxxii., p. 214; Brewster, 'Treaties on Magnetism', 1837, p. 111; Baumgartner, in the 'Zeitschrift fur Phys. und Mathem.', bd. ii., s. 419.

Almost all substances show themselves to be, in a certain degree, magnetic when they are conductors, that is to say, when a current of electricity is passing through them.

Although the knowledge of the attracting power of native iron magnets or loadstones appears to be of very ancient date among the nations of the West, there is strong historical evidence in proof of the striking fact that the knowledge of the directive power of a magnetic needle and of its relation to terrestrial magnetism was peculiar to the Chinese, a people living in the extremest eastern portions of Asia. More than a thousand years before our era, in the obscure age of Codrus, and about the time of the return of the Heraclidae to the Peloponnesus, the Chinese had already magnetic carriages, on which the movable arm of the figure of a man continually pointed to the south, as a guide by which to find the way across the boundless grass plains of Tartary; nay, even in the third century of our era, therefore at least 700 years before the use of the mariner's compass in European seas, Chinese vessels navigated the Indian Ocean* under the direction of magnetic needles pointing to the south.

[footnote] *Humboldt, 'Examen Critique de l'Hist. de la Geographie', t. iii., p. 36.

I have shown, in another work, what advantages this means of topographical direction, and the early knowledge and application of the magnetic needle gave the Chinese geographers over the Greeks and Romans, to whom, for instance, even the true direction of the Apennines and Pyrenees always remained unknown.*

[footnote] *'Asie Centrale', t. i., Introduction, p. xxxviii-xlii. The Western nations, the Greeks and the Romans, knew that magnetism could be communicated to iron, 'and that that metal would retain it for a length of time'. ("Sola haec materia ferri vires, a maguete lapide accipit, 'retinetque longo tempore." Plin., xxxiv., 14.) The great discovery of the terrestrial directive force depended, therefore, alone on this, that no one in the West had happened to observe an elongated fragment of magnetic iron stone, or a magnetic iron rod, floating, by the aid of a piece of wood, in water, or suspended in the air by a thread, in such a position as to admit of free motion.