How It Works (part 2)

This may be described as a valveless air-pump. Fig. 168 is a section of a "Gem" air-gun, with the mechanism set ready for firing. In the stock of the gun is the cylinder, in which an accurately fitting and hollow piston moves. A powerful helical spring, turned out of a solid bar of steel, is compressed between the inside end of the piston and the upper end of the butt. To set the gun, the catch is pressed down so that its hooked end disengages from the stock, and the barrel is bent downwards on pivot P. This slides the lower end of the compressing lever towards the butt, and a projection on the guide B, working in a groove, takes the piston with it. When the spring has been fully compressed, the triangular tip of the rocking cam R engages with a groove in the piston's head, and prevents recoil when the barrel is returned to its original position. On pulling the trigger, the piston is released and flies up the cylinder with great force, and the air in the cylinder is compressed and driven through the bore of the barrel, blocked by the leaden slug, to which the whole energy of the expanding spring is transmitted through the elastic medium of the air.

There are several other good types of air-gun, all of which employ the principles described above.

THE SELF-CLOSING DOOR-STOP

is another interesting pneumatic device. It consists of a cylinder with an air-tight piston, and a piston rod working through a cover at one end. The other end of the cylinder is pivoted to the door frame. When the door is opened the piston compresses a spring in the cylinder, and air is admitted past a cup leather on the piston to the upper part of the cylinder. This air is confined by the cup leather when the door is released, and escapes slowly through a leak, allowing the spring to regain its shape slowly, and by the agency of the piston rod to close the door.

THE ACTION OF WIND ON OBLIQUE SURFACES.

Why does a kite rise? Why does a boat sail across the wind? We can supply an answer almost instinctively in both cases, "Because the wind pushes the kite or sail aside." It will, however, be worth while to look for a more scientific answer. The kite cannot travel in the direction of the wind because it is confined by a string. But the face is so attached to the string that it inclines at an angle to the direction of the wind. Now, when a force meets an inclined surface which it cannot carry along with it, but which is free to travel in another direction, the force may be regarded as resolving itself into two forces, coming from each side of the original line. These are called the component forces.

To explain this we give a simple sketch of a kite in the act of flying (Fig. 169). The wind is blowing in the direction of the solid arrow A. The oblique surface of the kite resolves its force into the two components indicated by the dotted arrows B and C. Of these C only has lifting power to overcome the force of gravity. The kite assumes a position in which force C and gravity counterbalance one another.

A boat sailing across the wind is acted on in a similar manner (Fig. 170). The wind strikes the sail obliquely, and would thrust it to leeward were it not for the opposition of the water. The force A is resolved into forces B and C, of which C propels the boat on the line of its axis. The boat can be made to sail even "up" the wind, her head being brought round until a point is reached at which the force B on the boat, masts, etc., overcomes the force C. The capability of a boat for sailing up wind depends on her "lines" and the amount of surface she offers to the wind.

THE BALLOON

is a pear-shaped bag—usually made of silk—filled with some gas lighter than air. The tendency of a heavier medium to displace a lighter drives the gas upwards, and with it the bag and the wicker-work car attached to a network encasing the bag. The tapering neck at the lower end is open, to permit the free escape of gas as the atmospheric pressure outside diminishes with increasing elevation. At the top of the bag is a wooden valve opening inwards, which can be drawn down by a rope passing up to it through the neck whenever the aeronaut wishes to let gas escape for a descent. He is able to cause a very rapid escape by pulling another cord depending from a "ripping piece" near the top of the bag. In case of emergency this is torn away bodily, leaving a large hole. The ballast (usually sand) carried enables him to maintain a state of equilibrium between the upward pull of the gas and the downward pull of gravity. To sink he lets out gas, to rise he throws out ballast; and this process can be repeated until the ballast is exhausted. The greatest height ever attained by aeronauts is the 7¼ miles, or 37,000 feet, of Messrs. Glaisher and Coxwell on September 5, 1862. The ascent nearly cost them their lives, for at an elevation of about 30,000 feet they were partly paralyzed by the rarefaction of the air, and had not Mr. Coxwell been able to pull the valve rope with his teeth and cause a descent, both would have died from want of air.

The flying-machine, which scientific engineers have so long been trying to produce, will probably be quite independent of balloons, and will depend for its ascensive powers on the action of air on oblique surfaces. Sir Hiram Maxim's experimental air-ship embodied the principles shown by Fig. 171. On a deck was mounted an engine, E, extremely powerful for its weight. This drove large propellers, S S. Large aeroplanes, of canvas stretched over light frameworks, were set up overhead, the forward end somewhat higher than the rear. The machine was run on rails so arranged as to prevent it rising. Unfortunately an accident happened at the first trial and destroyed the machine.

In actual flight it would be necessary to have a vertical rudder for altering the horizontal direction, and a horizontal "tail" for steering up or down. The principle of an aeroplane is that of the kite, with this difference, that, instead of moving air striking a captive body, a moving body is propelled against more or less stationary air. The resolution of forces is shown by the arrows as before.

Up to the present time no practical flying-machine has appeared. But experimenters are hard at work examining the conditions which must be fulfilled to enable man to claim the "dominion of the air."

Chapter XVIII.

Hydraulic Machinery.

The siphon—The bucket pump—The force-pump—The most marvellous pump—The blood channels—The course of the blood—The hydraulic press—Household water-supply fittings—The ball-cock—The water-meter—Water-supply systems—The household filter—Gas traps—Water engines—The cream separator—The "hydro."

In the last chapter we saw that the pressure of the atmosphere is 15 lbs. to the square inch. Suppose that to a very long tube having a sectional area of one square inch we fit an air-tight piston (Fig. 172), and place the lower end of the tube in a vessel of water. On raising the piston a vacuum would be created in the tube, did not the pressure of the atmosphere force water up into the tube behind the piston. The water would continue to rise until it reached a point 34 feet perpendicularly above the level of the water in the vessel. The column would then weigh 15 lbs., and exactly counterbalance the atmospheric pressure; so that a further raising of the piston would not raise the water any farther. At sea-level, therefore, the lifting power of a pump by suction is limited to 34 feet. On the top of a lofty mountain, where the air-pressure is less, the height of the column would be diminished—in fact, be proportional to the pressure.

THE SIPHON

is an interesting application of the principle of suction. By its own weight water may be made to lift water through a height not exceeding 34 feet. This is explained by Fig. 173. The siphon pipe, A B C D, is in the first instance filled by suction. The weight of the water between A and B counter-balances that between B and C. But the column C D hangs, as it were, to the heels of B C, and draws it down. Or, to put it otherwise, the column B D, being heavier than the column B A, draws it over the topmost point of the siphon. Any parting between the columns, provided that B A does not exceed 34 feet, is impossible, as the pressure of the atmosphere on the mouth of B A is sufficient to prevent the formation of a vacuum.

THE BUCKET PUMP.

We may now pass to the commonest form of pump used in houses, stables, gardens, etc. (Fig. 174). The piston has a large hole through it, over the top of which a valve is hinged. At the bottom of the barrel is a second valve, also opening upwards, seated on the top of the supply pipe. In sketch (a) the first upstroke is in progress. A vacuum forms under the piston, or plunger, and water rises up the barrel to fill it. The next diagram (b) shows the first downstroke. The plunger valve now opens and allows water to rise above the piston, while the lower closes under the pressure of the water above and the pull of that below. During the second upstroke (c) the water above the piston is raised until it overflows through the spout, while a fresh supply is being sucked in below.

THE FORCE-PUMP.

For driving water to levels above that of the pump a somewhat different arrangement is required. One type of force-pump is shown in Figs. 175, 176. The piston now is solid, and the upper valve is situated in the delivery pipe. During an upstroke this closes, and the other opens; the reverse happening during a downstroke. An air-chamber is generally fitted to the delivery pipe when water is to be lifted to great heights or under high pressure. At each delivery stroke the air in the chamber is compressed, absorbing some of the shock given to the water in the pipe by the water coming from the pump; and its expansion during the next suction stroke forces the water gradually up the pipe. The air-chamber is a very prominent feature of the fire-engine.

A double-action force-pump is seen in Fig. 177, making an upward stroke. Both sides of the piston are here utilized, and the piston rod works through a water-tight stuffing-box. The action of the pump will be easily understood from the diagram.

THE MOST MARVELLOUS PUMP

known is the heart. We give in Fig. 178 a diagrammatic sketch of the system of blood circulation in the human body, showing the heart, the arteries, and the veins, big and little. The body is supposed to be facing the reader, so that the left lung, etc., is to his right.

The heart, which forces the blood through the body, is a large muscle (of about the size of the clenched fist) with four cavities. These are respectively known as the right and left auricles, and the right and left ventricles. They are arranged in two pairs, the auricle uppermost, separated by a fleshy partition. Between each auricle and its ventricle is a valve, which consists of strong membranous flaps, with loose edges turned downwards. The left-side valve is the mitral valve, that between the right auricle and ventricle the tricuspid valve. The edges of the valves fall together when the heart contracts, and prevent the passage of blood. Each ventricle has a second valve through which it ejects the blood. (That of the right ventricle has been shown double for the sake of convenience.)

The action of the heart is this:—The auricles and ventricles expand; blood rushes into the auricles from the channels supplying them, and distends them and the ventricles; the auricles contract and fill the ventricles below quite full (there are no valves above the auricles, but the force of contraction is not sufficient to return the blood to the veins); the ventricles contract; the mitral and tricuspid valves close; the valves leading to the arteries open; blood is forced out of the ventricles.

THE BLOOD CHANNELS

are of two kinds—(1) The arteries, which lead the blood into the circulatory system; (2) the veins, which lead the blood back to the heart. The arteries divide up into branches, and these again divide into smaller and smaller arteries. The smallest, termed capillaries (Latin, capillus, a hair), are minute tubes having an average diameter of 1⁄3000th of an inch. These permeate every part of the body. The capillary arteries lead into the smallest veins, which unite to form larger and larger veins, until what we may call the main streams are reached. Through these the blood flows to the heart.

There are three main points of difference between arteries and veins. In the first place, the larger arteries have thick elastic walls, and maintain their shape even when empty. This elasticity performs the function of the air-chamber of the force-pump. When the ventricles contract, driving blood into the arteries, the walls of the latter expand, and their contraction pushes the blood steadily forward without shock. The capillaries have very thin walls, so that fluids pass through them to and from the body, feeding it and taking out waste matter. The veins are all thin-walled, and collapse when empty. Secondly, most veins are furnished with valves, which prevent blood flowing the wrong way. These are similar in principle to those of the heart. Arteries have no valves. Thirdly, arteries are generally deeply set, while many of the veins run near the surface of the body. Those on the front of the arm are specially visible. Place your thumb on them and run it along towards the wrist, and you will notice that the veins distend owing to the closing of the valves just mentioned.

Arterial blood is red, and comes out from a cut in gulps, on account of the contraction of the elastic walls. If you cut a vein, blue blood issues in a steady stream. The change of colour is caused by the loss of oxygen during the passage of the blood through the capillaries, and the absorption of carbon dioxide from the tissues.

The lungs are two of the great purifiers of the blood. As it circulates through them, it gives up the carbon dioxide which it has absorbed, and receives pure oxygen in exchange. If the air of a room is "foul," the blood does not get the proper amount of oxygen. For this reason it is advisable for us to keep the windows of our rooms open as much as possible both day and night. Fatigue is caused by the accumulation of carbon dioxide and other impurities in the blood. When we run, the heart pumps blood through the lungs faster than they can purify it, and eventually our muscles become poisoned to such an extent that we have to stop from sheer exhaustion.

THE COURSE OF THE BLOOD.

It takes rather less than a minute for a drop of blood to circulate from the heart through the whole system and back to the heart.

We may briefly summarize the course of the circulation of the blood thus:—It is expelled from the left ventricle into the aorta and the main arteries, whence it passes into the smaller arteries, and thence into the capillaries of the brain, stomach, kidneys, etc. It here imparts oxygen to the body, and takes in impurities. It then enters the veins, and through them flows back to the right auricle; is driven into the right ventricle; is expelled into the pulmonary (lung) arteries; enters the lungs, and is purified. It returns to the left auricle through the pulmonary veins; enters the left auricle, passes to left ventricle, and so on.

A healthy heart beats from 120 times per minute in a one-year-old infant to 60 per minute in a very aged person. The normal rate for a middle-aged adult is from 80 to 70 beats.

Heart disease signifies the failure of the heart valves to close properly. Blood passes back when the heart contracts, and the circulation is much enfeebled. By listening through a stethoscope the doctor is able to tell whether the valves are in good order. A hissing sound during the beat indicates a leakage past the valves; a thump, or "clack," that they shut completely.

THE HYDRAULIC PRESS.

It is a characteristic of fluids and gases that if pressure be brought to bear on any part of a mass of either class of bodies it is transmitted equally and undiminished in all directions, and acts with the same force on all equal surfaces, at right angles to those surfaces. The great natural philosopher Pascal first formulated this remarkable fact, of which a simple illustration is given in Fig. 179. Two cylinders, A and B, having a bore of one and two inches respectively, are connected by a pipe. Water is poured in, and pistons fitting the cylinders accurately and of equal weight are inserted. On piston B is placed a load of 10 lbs. To prevent A rising above the level of B, it must be loaded proportionately. The area of piston A is four times that of B, so that if we lay on it a 40-lb. weight, neither piston will move. The walls of the cylinders and connecting pipe are also pressed outwards in the ratio of 10 lbs. for every part of their interior surface which has an area equal to that of piston B.

The hydraulic press is an application of this law. Cylinder B is represented by a force pump of small bore, capable of delivering water at very high pressures (up to 10 tons per square inch). In the place of A we have a stout cylinder with a solid plunger, P (Fig. 180), carrying the table on which the object to be pressed is placed. Bramah, the inventor of the hydraulic press, experienced great difficulty in preventing the escape of water between the top of the cylinder and the plunger. If a "gland" packing of the type found in steam-cylinders were used, it failed to hold back the water unless it were screwed down so tightly as to jam the plunger. He tried all kinds of expedients without success; and his invention, excellent though it was in principle, seemed doomed to failure, when his foreman, Henry Maudslay, solved the problem in a simple but most masterly manner. He had a recess turned in the neck of the cylinder at the point formerly occupied by the stuffing-box, and into this a leather collar of U-section (marked solid black in Fig. 180) was placed with its open side downwards. When water reached it, it forced the edges apart, one against the plunger, the other against the walls of the recess, with a degree of tightness proportionate to the pressure. On water being released from the cylinder the collar collapsed, allowing the plunger to sink without friction.

The principle of the hydraulic press is employed in lifts; in machines for bending, drilling, and riveting steel plates, or forcing wheels on or off their axles; for advancing the "boring shield" of a tunnel; and for other purposes too numerous to mention.

HOUSEHOLD WATER-SUPPLY FITTINGS.

Among these, the most used is the tap, or cock. When a house is served by the town or district water supply, the fitting of proper taps on all pipes connected with the supply is stipulated for by the water-works authorities. The old-fashioned "plug" tap is unsuitable for controlling high-pressure water on account of the suddenness with which it checks the flow. Lest the reader should have doubts as to the nature of a plug tap, we may add that it has a tapering cone of metal working in a tapering socket. On the cone being turned till a hole through it is brought into line with the channel of the tap, water passes. A quarter turn closes the tap.

Its place has been taken by the screw-down cock. A very common and effective pattern is shown in Fig. 181. The valve V, with a facing of rubber, leather, or some other sufficiently elastic substance, is attached to a pin, C, which projects upwards into the spindle A of the tap. This spindle has a screw thread on it engaging with a collar, B. When the spindle is turned it rises or falls, allowing the valve to leave its seating, V S, or forcing it down on to it. A packing P in the neck of B prevents the passage of water round the spindle. To open or close the tap completely is a matter of several turns, which cannot be made fast enough to produce a "water-hammer" in the pipes by suddenly arresting the flow. The reader will easily understand that if water flowing at the rate of several miles an hour is abruptly checked, the shock to the pipes carrying it must be very severe.

THE BALL-COCK

is used to feed a cistern automatically with water, and prevent the water rising too far in the cistern (Fig. 182). Water enters the cistern through a valve, which is opened and closed by a plug faced with rubber. The lower extremity of the plug is flattened, and has a rectangular hole cut in it. Through this passes a lever, L, attached at one end to a hollow copper sphere, and pivoted at the other on the valve casing. This casing is not quite circular in section, for two slots are cast in the circumference to allow water to pass round the plug freely when the valve is open. The buoyancy of the copper sphere is sufficient to force the plug's face up towards its seating as the valve rises, and to cut off the supply entirely when a certain level has been attained. If water is drawn off, the sphere sinks, the valve opens, and the loss is made good.

THE WATER-METER.

Some consumers pay a sum quarterly for the privilege of a water supply, and the water company allows them to use as much as they require. Others, however, prefer to pay a fixed amount for every thousand gallons used. In such cases, a water-meter is required to record the consumption. We append a sectional diagram of Kennedy's patent water-meter (Fig. 183), very widely used. At the bottom is the measuring cylinder, fitted with a piston, (6), which is made to move perfectly water-tight and free from friction by means of a cylindrical ring of india-rubber, rolling between the body of the piston and the internal surface of the cylinder. The piston rod (25), after passing through a stuffing-box in the cylinder cover, is attached to a rack, (15), which gears with a cog, (13), fixed on a shaft. As the piston moves up and down, this cog is turned first in one direction, then in the other. To this shaft is connected the index mechanism (to the right). The cock-key (24) is so constructed that it can put either end of the measuring cylinder in communication with the supply or delivery pipes, if given a quarter turn (see Fig. 184). The weighted lever (14) moves loosely on the pinion shaft through part of a circle. From the pinion project two arms, one on each side of the lever. When the lever has been lifted by one of these past the vertical position, it falls by its own weight on to a buffer-box rest, (18). In doing so, it strikes a projection on the duplex lever (19), which is joined to the cock-key, and gives the latter a quarter turn.

In order to follow the working of the meter, we must keep an eye on Figs. 183 and 184 simultaneously. Water is entering from A, the supply pipe. It flows through the cock downwards through channel D into the lower half of the cylinder. The piston rises, driving out the water above it through C to the delivery pipe B. Just as the piston completes its stroke the weight, raised by the rack and pinion, topples over, and strikes the key-arm, which it sends down till stopped by the buffer-box. The tap is then at right angles to the position shown in Fig. 184, and water is directed from A down C into the top of the cylinder, forcing the piston down, while the water admitted below during the last stroke is forced up the passage D, and out by the outlet B. Before the piston has arrived at the bottom of the cylinder, the lifter will have lifted the weighted lever from the buffer-box, and raised it to a vertical position; from there it will have fallen on the right-hand key-arm, and have brought the cock-key to its former position, ready to begin another upward stroke.

The index mechanism makes allowance for the fact that the bevel-wheel on the pinion shaft has its direction reversed at the beginning of every stroke of the piston. This bevel engages with two others mounted loosely on the little shaft, on which is turned a screw thread to revolve the index counter wheels. Each of these latter bevels actuates the shaft through a ratchet; but while one turns the shaft when rotating in a clockwise direction only, the other engages it when making an anti-clockwise revolution. The result is that the shaft is always turned in the same direction.

WATER-SUPPLY SYSTEMS.

The water for a town or a district supply is got either from wells or from a river. In the former case it may be assumed to be free from impurities. In the latter, there is need for removing all the objectionable and dangerous matter which river water always contains in a greater or less degree. This purification is accomplished by first leading the water into large settling tanks, where the suspended matter sinks to the bottom. The water is then drawn off into filtration beds, made in the following manner. The bottom is covered with a thick layer of concrete. On this are laid parallel rows of bricks, the rows a small distance apart. Then come a layer of bricks or tiles placed close together; a layer of coarse gravel; a layer of finer gravel; and a thick layer of sand at the top. The sand arrests any solid matter in the water as it percolates to the gravel and drains below. Even the microbes, of microscopic size, are arrested as soon as the film of mud has formed on the top of the sand. Until this film is formed the filter is not in its most efficient condition. Every now and then the bed is drained, the surface mud and sand carefully drained off, and fresh sand put in their place. A good filter bed should not pass more than from two to three gallons per hour for every square foot of surface, and it must therefore have a large area.

It is sometimes necessary to send the water through a succession of beds, arranged in terraces, before it is sufficiently pure for drinking purposes.

THE HOUSEHOLD FILTER.

When there is any doubt as to the wholesomeness of the water supply, a small filter is often used. The microbe-stopper is usually either charcoal, sand, asbestos, or baked clay of some kind. In Fig. 185 we give a section of a Maignen filter. R is the reservoir for the filtered water; A the filter case proper; D a conical perforated frame; B a jacket of asbestos cloth secured top and bottom by asbestos cords to D; C powdered carbon, between which and the asbestos is a layer of special chemical filtering medium. A perforated cap, E, covers in the carbon and prevents it being disturbed when water is poured in. The carbon arrests the coarser forms of matter; the asbestos the finer. The asbestos jacket is easily removed and cleansed by heating over a fire.

The most useful form of household filter is one which can be attached to a tap connected with the main. Such a filter is usually made of porcelain or biscuit china. The Berkefeld filter has an outer case of iron, and an interior hollow "candle" of porcelain from which a tube passes through the lid of the filter to a storage tank for the filtered water. The water from the main enters the outer case, and percolates through the porcelain walls to the internal cavity and thence flows away through the delivery pipe.

Whatever be the type of filter used it must be cleansed at proper intervals. A foul filter is very dangerous to those who drink the water from it. It has been proved by tests that, so far from purifying the water, an inefficient and contaminated filter passes out water much more highly charged with microbes than it was before it entered. We must not therefore think that, because water has been filtered, it is necessarily safe. The reverse is only too often the case.

GAS TRAPS.

Dangerous microbes can be breathed as well as drunk into the human system. Every communication between house and drains should be most carefully "trapped." The principle of a gas trap between, say, a kitchen sink and the drain to carry off the water is given in Fig. 186. Enough water always remains in the bend to rise above the level of the elbow, effectually keeping back any gas that there may be in the pipe beyond the bend.

WATER-ENGINES.

Before the invention of the steam-engine human industries were largely dependent on the motive power of the wind and running water. But when the infant nursed by Watt and Stephenson had grown into a giant, both of these natural agents were deposed from the important position they once held. Windmills in a state of decay crown many of our hilltops, and the water-wheel which formerly brought wealth to the miller now rots in its mountings at the end of the dam. Except for pumping and moving boats and ships, wind-power finds its occupation gone. It is too uncertain in quantity and quality to find a place in modern economics. Water-power, on the other hand, has received a fresh lease of life through the invention of machinery so scientifically designed as to use much more of the water's energy than was possible with the old-fashioned wheel.

The turbine, of which we have already spoken in our third chapter, is now the favourite hydraulic engine. Some water-turbines work on much the same principle as the Parsons steam-turbine; others resemble the De Laval. Among the latter the Pelton wheel takes the first place. By the courtesy of the manufacturers we are able to give some interesting details and illustrations of this device.

The wheel, which may be of any diameter from six inches to ten feet, has buckets set at regular intervals round the circumference, sticking outwards. Each bucket, as will be gathered from our illustration of an enormous 5,000 h.p. wheel (Fig. 187), is composed of two cups. A nozzle is so arranged as to direct water on the buckets just as they reach the lowest point of a revolution (see Fig. 188). The water strikes the bucket on the partition between the two cups, which turns it right and left round the inside of the cups. The change of direction transfers the energy of the water to the wheel.

The speed of the wheel may be automatically regulated by a deflecting nozzle (Fig. 189), which has a ball and socket joint to permit of its being raised or lowered by a centrifugal governor, thus throwing the stream on or off the buckets. The power of the wheel is consequently increased or diminished to meet the change of load, and a constant speed is maintained. When it is necessary to waste as little water as possible, a concentric tapered needle may be fitted inside the nozzle. When the nozzle is in its highest position the needle tip is withdrawn; as the nozzle sinks the needle protrudes, gradually decreasing the discharge area of the nozzle.

Pelton wheels are designed to run at all speeds and to use water of any pressure. At Manitou, Colorado, is an installation of three wheels operated by water which leaves the nozzle at the enormous pressure of 935 lbs. per square inch. It is interesting to note that jets of very high-pressure water offer astonishing resistance to any attempt to deflect their course. A three-inch jet of 500-lb. water cannot be cut through by a blow from a crowbar.

In order to get sufficient pressure for working hydraulic machinery in mines, factories, etc., water is often led for many miles in flumes, or artificial channels, along the sides of valleys from the source of supply to the point at which it is to be used. By the time that point is reached the difference between the gradients of the flume and of the valley bottom has produced a difference in height of some hundreds of feet.

The full-page illustration on p. 380 affords a striking testimony to the wonderful progress made in engineering practice during the last fifty years. The huge water-wheel which forms the bulk of the picture is that at Laxey, in the Isle of Man. It is 72½ feet in diameter, and is supposed to develop 150 horse-power, which is transmitted several hundreds of feet by means of wooden rods supported at regular intervals. The power thus transmitted operates a system of pumps in a lead mine, raising 250 gallons of water per minute, to an elevation of 1,200 feet. The driving water is brought some distance to the wheel in an underground conduit, and is carried up the masonry tower by pressure, flowing over the top into the buckets on the circumference of the wheel.

The little cut in the upper corner represents a Pelton wheel drawn on the same scale, which, given an equal supply of water at the same pressure, would develop the same power as the Laxey monster. By the side of the giant the other appears a mere toy.

THE CREAM SEPARATOR.

In 1864 Denmark went to war with Germany, and emerged from the short struggle shorn of the provinces of Lauenburg, Holstein, and Schleswig. The loss of the two last, the fairest and most fertile districts of the kingdom, was indeed grievous. The Danish king now ruled only over a land consisting largely of moor, marsh, and dunes, apparently worthless for any purpose. But the Danes, with admirable courage, entered upon a second struggle, this time with nature. They made roads and railways, dug irrigation ditches, and planted forest trees; and so gradually turned large tracts of what had been useless country into valuable possessions. Agriculture being much depressed, owing to the low price of corn, they next gave their attention to the improvement of dairy farming. Labour-saving machinery of all kinds was introduced, none more important than the device for separating the fatty from the watery constituents of milk. It would not be too much to say that the separator is largely responsible for the present prosperity of Denmark.

How does it work? asks the reader. Centrifugal force is the governing principle. To explain its application we append a sectional illustration (Fig. 191) of Messrs. Burmeister and Wain's hand-power separator, which may be taken as generally representative of this class of machines. Inside a circular casing is a cylindrical bowl, D, mounted on a shaft which can be revolved 5,000 times a minute by means of the cog-wheels and the screw thread chased on it near the bottom extremity. Milk flows from the reservoir R (supported on a stout arm) through tap A into a little distributer on the top of the separator, and from it drops into the central tube C of the bowl. Falling to the bottom, it is flung outwards by centrifugal force, finds an escape upwards through the holes a a, and climbs up the perforated grid e, the surface of which is a series of pyramidical excrescences, and finally reaches the inner surface of the drum proper. The velocity of rotation is so tremendous that the heavier portions of the milk—that is, the watery—crowd towards the point furthest from the centre, and keep the lighter fatty elements away from contact with the sides of the drum. In the diagram the water is represented by small circles, the cream by small crosses.

As more milk enters the drum it forces upwards what is already there. The cap of the drum has an inner jacket, F, which at the bottom all but touches the side of the drum. The distance between them is the merest slit; but the cream is deflected up outside F into space E, and escapes through a hole one-sixteenth of an inch in diameter perforating the plate G. The cream is flung into space K and trickles out of spout B, while the water flies into space H and trickles away through spout A.

THE "HYDRO.,"

used in laundries for wringing clothes by centrifugal force, has a solid outer casing and an inner perforated cylindrical cage, revolved at high speed by a vertical shaft. The wet clothes are placed in the cage, and the machine is started. The water escapes through the perforations and runs down the side of the casing to a drain. After a few minutes the clothes are dry enough for ironing. So great is the centrifugal force that they are consolidated against the sides of the cage, and care is needed in their removal.

Chapter XIX.

Heating and Lighting.

The hot-water supply—The tank system—The cylinder system—How a lamp works—Gas and gasworks—Automatic stoking—A gas governor—The gas meter—Incandescent gas lighting.

HOT-WATER SUPPLY.

A well-equipped house is nowadays expected to contain efficient apparatus for supplying plenty of hot water at all hours of the day. There is little romance about the kitchen boiler and the pipes which the plumber and his satellites have sometimes to inspect and put right, but the methods of securing a proper circulation of hot water through the house are sufficiently important and interesting to be noticed in these pages.

In houses of moderate size the kitchen range does the heating. The two systems of storing and distributing the heated water most commonly used are—(1) The tank system; (2) the cylinder system.

THE TANK SYSTEM

is shown diagrammatically in Fig. 192. The boiler is situated at the back of the range, and when a "damper" is drawn the fire and hot gases pass under it to a flue leading to the chimney. The almost boiling water rises to the top of the boiler and thence finds its way up the flow pipe into the hot-water tank A, displacing the somewhat colder water there, which descends through the return pipe to the bottom of the boiler.

Water is drawn off from the flow pipe. This pipe projects some distance through the bottom of A, so that the hottest portion of the contents may be drawn off first. A tank situated in the roof, and fed from the main by a ball-cock valve, communicates with A through the siphon pipe S. The bend in this pipe prevents the ascent of hot water, which cannot sink through water colder than itself. From the top of A an expansion pipe is led up and turned over the cold-water tank to discharge any steam which may be generated in the boiler.

A hot-water radiator for warming the house may be connected to the flow and return pipes as shown. Since it opens a "short circuit" for the circulation, the water in the tank above will not be so well heated while it is in action. If cocks are fitted to the radiator pipes, the amount of heat thus deflected can be governed.

A disadvantage of the tank system is that the tank, if placed high enough to supply all flows, is sometimes so far from the boiler that the water loses much of its heat in the course of circulation. Also, if for any reason the cold water fails, tank A may be entirely emptied, circulation cease, and the water in the boiler and pipes boil away rapidly.

THE CYLINDER SYSTEM

(Fig. 193) is open to neither of these objections. Instead of a rectangular tank up aloft, we now have a large copper cylinder situated in the kitchen near the range. The flow and return pipes are continuous, and the cold supply enters the bottom of the cylinder through a pipe with a siphon bend in it. As before, water is drawn off from the flow pipe, and a radiator may be put in the circuit. Since there is no draw-off point below the top of the cylinder, even if the cold supply fails the cylinder will remain full, and the failure will be discovered long before there is any danger of the water in it boiling away.

Boiler explosions are due to obstructions in the pipes. If the expansion pipe and the cold-water supply pipe freeze, there is danger of a slight accumulation of steam; and if one of the circulation pipes is also blocked, steam must generate until "something has to go," which is naturally the boiler. Assuming that the pipes are quite full to the points of obstruction, the fracture would result from the expansion of the water. Steam cannot generate unless there be a space above the water. But the expanding water has stored up the heat which would have raised steam, and the moment expansion begins after fracture this energy is suddenly let loose. Steam forms instantaneously, augmenting the effects of the explosion. From this it will be gathered that all pipes should be properly protected against frost; especially near the roof.

Another cause of disaster is the furring up of the pipes with the lime deposited by hard water when heated. When hard water is used, the pipes will sooner or later be blocked near the boiler; and as the deposit is too hard to be scraped away, periodical renewals are unavoidable.

HOW A LAMP WORKS.

From heating we turn to lighting, and first to the ordinary paraffin lamp. The two chief things to notice about this are the wick and the chimney. The wick, being made of closely-woven cotton, draws up the oil by what is known as capillary attraction. If you dip the ends of two glass tubes, one half an inch, the other one-eighth of an inch in diameter, into a vessel of water, you will notice that the water rises higher in the smaller tube. Or get two clean glass plates and lay them face to face, touching at one end, but kept slightly apart at the other by some small object. If they are partly submerged perpendicularly, the water will rise between the plates—furthest on the side at which the two plates touch, and less and less as the other edge is approached. The tendency of liquids to rise through porous bodies is a phenomenon for which we cannot account.

Mineral oil contains a large proportion of carbon and hydrogen; it is therefore termed hydro-carbon. When oil reaches the top of a lighted wick, the liquid is heated until it turns into gas. The carbon and hydrogen unite with the oxygen of the air. Some particles of the carbon apparently do not combine at once, and as they pass through the fiery zone of the flame are heated to such a temperature as to become highly luminous. It is to produce these light-rays that we use a lamp, and to burn our oil efficiently we must supply the flame with plenty of oxygen, with more than it could naturally obtain. So we surround it with a transparent chimney of special glass. The air inside the chimney is heated, and rises; fresh air rushes in at the bottom, and is also heated and replaced. As the air passes through, the flame seizes on the oxygen. If the wick is turned up until the flame becomes smoky and flares, the point has been passed at which the induced chimney draught can supply sufficient oxygen to combine with the carbon of the vapour, and the "free" carbon escapes as smoke.

The blower-plate used to draw up a fire (Fig. 194) performs exactly the same function as the lamp chimney, but on a larger scale. The plate prevents air passing straight up the chimney over the coals, and compels it to find a way through the fire itself to replace the heated air rising up the chimney.

GAS AND GASWORKS.

A lamp is an apparatus for converting hydro-carbon mineral oil into gas and burning it efficiently. The gas-jet burns gases produced by driving off hydro-carbon vapours from coal in apparatus specially designed for the purpose. Gas-making is now, in spite of the competition of electric lighting, so important an industry that we shall do well to glance at the processes which it includes. Coal gas may be produced on a very small scale as follows:—Fill a tin canister (the joints of which have been made by folding the metal, not by soldering) with coal, clap on the lid, and place it, lid downwards, in a bright fire, after punching a hole in the bottom. Vapour soon begins to issue from the hole. This is probably at first only steam, due to the coal being more or less damp. But if a lighted match be presently applied the vapour takes fire, showing that coal gas proper is coming off. The flame lasts for a long time. When it dies the canister may be removed and the contents examined. Most of the carbon remains in the form of coke. It is bulk for bulk much lighter than coal, for the hydrogen, oxygen, and other gases, and some of the carbon have been driven off by the heat. The coke itself burns if placed in a fire, but without any smoke, such as issues from coal.

Our home-made gas yields a smoky and unsatisfactory flame, owing to the presence of certain impurities—ammonia, tar, sulphuretted hydrogen, and carbon bisulphide. A gas factory must be equipped with means of getting rid of these objectionable constituents. Turning to Fig. 195, which displays very diagrammatically the main features of a gas plant, we observe at the extreme right the retorts, which correspond to our canister. These are usually long fire-brick tubes of D-section, the flat side at the bottom. Under each is a furnace, the flames of which play on the bottom, sides, and inner end of the retort. The outer end projecting beyond the brickwork seating has an iron air-tight door for filling the retort through, immediately behind which rises an iron exit pipe, A, for the gases. Tar, which vaporizes at high temperatures, but liquefies at ordinary atmospheric heat, must first be got rid of. This is effected by passing the gas through the hydraulic main, a tubular vessel half full of water running the whole length of the retorts. The end of pipe A dips below the surface of the water, which condenses most of the tar and steam. The partly-purified gas now passes through pipe B to the condensers, a series of inverted U-pipes standing on an iron chest with vertical cross divisions between the mouths of each U. These divisions dip into water, so that the gas has to pass up one leg of a U, down the other, up the first leg of the second pipe, and so on, till all traces of the tar and other liquid constituents have condensed on the inside of the pipe, from which they drop into the tank below.

The next stage is the passage of the scrubber, filled with coke over which water perpetually flows. The ammonia gas is here absorbed. There still remain the sulphuretted hydrogen and the carbon bisulphide, both of which are extremely offensive to the nostrils. Slaked lime, laid on trays in an air-tight compartment called the lime purifier, absorbs most of the sulphurous elements of these; and the coal gas is then fit for use. On leaving the purifiers it flows into the gasometer, or gasholder, the huge cake-like form of which is a very familiar object in the environs of towns. The gasometer is a cylindrical box with a domed top, but no bottom, built of riveted steel plates. It stands in a circular tank of water, so that it may rise and fall without any escape of gas. The levity of the gas, in conjunction with weights attached to the ends of chains working over pulleys on the framework surrounding the holder, suffices to raise the holder.

Some gasometers have an enormous capacity. The record is at present held by that built for the South Metropolitan Gas Co., London, by Messrs. Clayton & Son of Leeds. This monster (of which we append an illustration, Fig. 196) is 300 feet in diameter and 180 feet high. When fully extended it holds 12,158,600 cubic feet of gas. Owing to its immense size, it is built on the telescopic principle in six "lifts," of 30 feet deep each. The sides of each lift, or ring, except the topmost, have a section shaped somewhat like the letter N. Two of the members form a deep, narrow cup to hold water, in which the "dip" member of the ring above it rises and falls.

AUTOMATIC STOKING.

The labour of feeding the retorts with coal and removing the coke is exceedingly severe. In the illustration on p. 400 (made from a very fine photograph taken by Mr. F. Marsh of Clifton) we see a man engaged in "drawing" the retorts through the iron doors at their outer ends. Automatic machinery is now used in large gasworks for both operations. One of the most ingenious stokers is the De Brouwer, shown at work in Fig. 198. The machine is suspended from an overhead trolley running on rails along the face of the retorts. Coal falls into a funnel at the top of the telescopic pipe P from hoppers in the story above, which have openings, H H, controlled by shutters. The coal as it falls is caught by a rubber belt working round part of the circumference of the large wheel W and a number of pulleys, and is shot into the mouth of the retort. The operator is seen pulling the handle which opens the shutter of the hopper above the feed-tube, and switching on the 4 h.p. electric motor which drives the belt and moves the machine about. One of these feeders will charge a retort 20 feet long in twenty-two seconds.

A GAS GOVERNOR.

Some readers may have noticed that late at night a gas-jet, which a few hours before burned with a somewhat feeble flame when the tap was turned fully on, now becomes more and more vigorous, and finally may flare up with a hissing sound. This is because many of the burners fed by the main supplying the house have been turned off, and consequently there is a greater amount of gas available for the jets still burning, which therefore feel an increased pressure. As a matter of fact, the pressure of gas in the main is constantly varying, owing partly to the irregularity of the delivery from the gasometer, and partly to the fact that the number of burners in action is not the same for many minutes together. It must also be remembered that houses near the gasometer end of the main will receive their gas at a higher pressure than those at the other end. The gas stored in the holders may be wanted for use in the street lamps a few yards away, or for other lamps several miles distant. It is therefore evident that if there be just enough pressure to give a good supply to the nearest lamp, there will be too little a short distance beyond it, and none at all at the extreme point; so that it is necessary to put on enough pressure to overcome the friction on all these miles of pipe, and give just enough gas at the extreme end. It follows that at all intermediate points the pressure is excessive. Gas of the average quality is burned to the greatest advantage, as regards its light-giving properties, when its pressure is equal to that of a column of water half an inch high, or about 1⁄50 lb. to the square inch. With less it gives a smoky, flickering light, and with more the combustion is also imperfect.

Every house supply should therefore be fitted with a gas governor, to keep the pressure constant. A governor frequently used, the Stott, is shown in section in Fig. 199. Gas enters from the main on the right, and passes into a circular elbow, D, which has top and bottom apertures closed by the valves V V. Attached to the valve shaft is a large inverted cup of metal, the tip of which is immersed in mercury. The pressure at which the governor is to act is determined by the weights W, with which the valve spindle is loaded at the top. As soon as this pressure is exceeded, the gas in C C lifts the metal cup, and V V are pressed against their seats, so cutting off the supply. Gas cannot escape from C C, as it has not sufficient pressure to force its way through the mercury under the lip of the cup. Immediately the pressure in C C falls, owing to some of the gas being used up, the valves open and admit more gas. When the fluctuations of pressure are slight, the valves never close completely, but merely throttle the supply until the pressure beyond them falls to its proper level—that is, they pass just as much gas as the burners in use can consume at the pressure arranged for.

Governors of much larger size, but working on much the same principle, are fitted to the mains at the point where they leave the gasometers. They are not, however, sensitive to local fluctuations in the pipes, hence the necessity for separate governors in the house between the meter and the burners.

THE GAS-METER

commonly used in houses acts on the principle shown in Fig. 200. The air-tight casing is divided by horizontal and vertical divisions into three gas-chambers, B, C, and D. Gas enters at A, and passes to the valve chamber B. The slide-valves of this allow it to pass into C and D, and also into the two circular leather bellows E, F, which are attached to the central division G, but are quite independent of one another.

We will suppose that in the illustration the valves are admitting gas to chamber C and bellows F. The pressure in C presses the circular head of E towards the division G, expelling the contents of the bellows through an outlet pipe (not shown) to the burners in operation within the house. Simultaneously the inflation of F forces the gas in chamber D also through the outlet. The head-plates of the bellows are attached to rods and levers (not shown) working the slide-valves in B. As soon as E is fully in, and F fully expanded, the valves begin to open and put the inlet pipe in communication with D and E, and allow the contents of F and C to escape to the outlet. The movements of the valve mechanism operate a train of counting wheels, visible through a glass window in the side of the case. As the bellows have a definite capacity, every stroke that they give means that a certain volume of gas has been ejected either from them or from the chambers in which they move: this is registered by the counter. The apparatus practically has two double-action cylinders (of which the bellows ends are the pistons) working on the same principle as the steam-cylinder (Fig. 21). The valves have three ports—the central, or exhaust, leading to the outlet, the outer ones from the inlet. The bellows are fed through channels in the division G.

INCANDESCENT GAS LIGHTING.

The introduction of the electric arc lamp and the incandescent glow-lamp seemed at one time to spell the doom of gas as an illuminating agent. But the appearance in 1886 of the Welsbach incandescent mantle for gas-burners opened a prosperous era in the history of gas lighting.

The luminosity of a gas flame depends on the number of carbon particles liberated within it, and the temperature to which these particles can be heated as they pass through the intensely hot outside zone of the flame. By enriching the gas in carbon more light is yielded, up to a certain point, with a flame of a given temperature. To increase the heat of the flame various devices were tried before the introduction of the incandescent mantle, but they were found to be too short-lived to have any commercial value. Inventors therefore sought for methods by which the emission of light could be obtained from coal gas independently of the incandescence of the carbon particles in the flame itself; and step by step it was discovered that gas could be better employed merely as a heating agent, to raise to incandescence substances having a higher emissivity of light than carbon.

Dr. Auer von Welsbach found that the substances most suitable for incandescent mantles were the oxides of certain rare metals, thorium, and cerium. The mantle is made by dipping a cylinder of cotton net into a solution of nitrate of thorium and cerium, containing 99 per cent. of the former and 1 per cent. of the latter metal. When the fibres are sufficiently soaked, the mantle is withdrawn, squeezed, and placed on a mould to dry. It is next held over a Bunsen gas flame and the cotton is burned away, while the nitrates are converted into oxides. The mantle is now ready for use, but very brittle. So it has to undergo a further dipping, in a solution of gun-cotton and alcohol, to render it tough enough for packing. When it is required for use, it is suspended over the burner by an asbestos thread woven across the top, a light is applied to the bottom, and the collodion burned off, leaving nothing but the heat-resisting oxides.

The burner used with a mantle is constructed on the Bunsen principle. The gas is mixed, as it emerges from the jet, with sufficient air to render its combustion perfect. All the carbon is burned, and the flame, though almost invisible, is intensely hot. The mantle oxides convert the heat energy of the flame into light energy. This is proved not only by the intense whiteness of the mantle, but by the fact that the heat issuing from the chimney of the burner is not nearly so great when the mantle is in position as when it is absent.

The incandescent mantle is more extensively used every year. In Germany 90 per cent. of gas lighting is on the incandescent system, and in England about 40 per cent. We may notice, as an interesting example of the fluctuating fortunes of invention, that the once doomed gas-burner has, thanks to Welsbach's mantle, in many instances replaced the incandescent electric lamps that were to doom it.

Chapter XX.

Various Mechanisms.

Clocks and Watches:—A short history of timepieces—The construction of timepieces—The driving power—The escapement—Compensating pendulums—The spring balance—The cylinder escapement—The lever escapement—Compensated balance-wheels—Keyless winding mechanism for watches—The hour hand train. Locks:—The Chubb lock—The Yale lock. The Cycle:—The gearing of a cycle—The free wheel—The change-speed gear. Agricultural Machines:—The threshing-machine—Mowing-machines. Some Natural Phenomena:—Why sun-heat varies in intensity—The tides—Why high tide varies daily.

CLOCKS AND WATCHES.

A SHORT HISTORY OF TIMEPIECES.

The oldest device for measuring time is the sun-dial. That of Ahaz mentioned in the Second Book of Kings is the earliest dial of which we have record. The obelisks of the Egyptians and the curious stone pillars of the Druidic age also probably served as shadow-casters.

The clepsydra, or water-clock, also of great antiquity, was the first contrivance for gauging the passage of the hours independently of the motion of the earth. In its simplest form it was a measure into which water fell drop by drop, hour levels being marked on the inside. Subsequently a very simple mechanism was added to drive a pointer—a float carrying a vertical rack, engaging with a cog on the pointer spindle; or a string from the float passed over a pulley attached to the pointer and rotated it as the float rose, after the manner of the wheel barometer (Fig. 153). In 807 A.D. Charlemagne received from the King of Persia a water-clock which struck the hours. It is thus described in Gifford's "History of France":—"The dial was composed of twelve small doors, which represented the division of the hours. Each door opened at the hour it was intended to represent, and out of it came a small number of little balls, which fell one by one, at equal distances of time, on a brass drum. It might be told by the eye what hour it was by the number of doors that were open, and by the ear by the number of balls that fell. When it was twelve o'clock twelve horsemen in miniature issued forth at the same time and shut all the doors."

Sand-glasses were introduced about 330 A.D. Except for special purposes, such as timing sermons and boiling eggs, they have not been of any practical value.

The clepsydra naturally suggested to the mechanical mind the idea of driving a mechanism for registering time by the force of gravity acting on some body other than water. The invention of the weight-driven clock is attributed, like a good many other things, to Archimedes, the famous Sicilian mathematician of the third century B.C.; but no record exists of any actual clock composed of wheels operated by a weight prior to 1120 A.D. So we may take that year as opening the era of the clock as we know it.

About 1500 Peter Hele of Nuremberg invented the mainspring as a substitute for the weight, and the watch appeared soon afterwards (1525 A.D.). The pendulum was first adopted for controlling the motion of the wheels by Christian Huygens, a distinguished Dutch mechanician, in 1659.

To Thomas Tompion, "the father of English watchmaking," is ascribed the honour of first fitting a hairspring to the escapement of a watch, in or about the year 1660. He also introduced the cylinder escapement now so commonly used in cheap watches. Though many improvements have been made since his time, Tompion manufactured clocks and watches which were excellent timekeepers, and as a reward for the benefits conferred on his fellows during his lifetime, he was, after death, granted the exceptional honour of a resting-place in Westminster Abbey.

THE CONSTRUCTION OF TIMEPIECES.

A clock or watch contains three main elements:—(1) The source of power, which may be a weight or a spring; (2) the train of wheels operated by the driving force; (3) the agent for controlling the movements of the train—this in large clocks is usually a pendulum, in small clocks and watches a hairspring balance. To these may be added, in the case of clocks, the apparatus for striking the hour.

THE DRIVING POWER.

Weights are used only in large clocks, such as one finds in halls, towers, and observatories. The great advantage of employing weights is that a constant driving power is exerted. Springs occupy much less room than weights, and are indispensable for portable timepieces. The employment of them caused trouble to early experimenters on account of the decrease in power which necessarily accompanies the uncoiling of a wound-up spring. Jacob Zech of Prague overcame the difficulty in 1525 by the invention of the fusee, a kind of conical pulley interposed between the barrel, or circular drum containing the mainspring, and the train of wheels which the spring has to drive. The principle of the "drum and fusee" action will be understood from Fig. 201. The mainspring is a long steel ribbon fixed at one end to an arbor (the watchmaker's name for a spindle or axle), round which it is tightly wound. The arbor and spring are inserted in the barrel. The arbor is prevented from turning by a ratchet, B, and click, and therefore the spring in its effort to uncoil causes the barrel to rotate.

A string of catgut (or a very fine chain) is connected at one end to the circumference of the drum, and wound round it, the other end being fixed to the larger end of the fusee, which is attached to the driving-wheel of the watch or clock by the intervention of a ratchet and click (not shown). To wind the spring the fusee is turned backward by means of a key applied to the square end A of the fusee arbor, and this draws the string from off the drum on to the fusee. The force of the spring causes the fusee to rotate by pulling the string off it, coil by coil, and so drives the train of wheels. But while the mainspring, when fully wound, turns the fusee by uncoiling the string from the smallest part of the fusee, it gets the advantage of the larger radius as its energy becomes lessened.

The fusee is still used for marine chronometers, for some clocks that have a mainspring and pendulum, and occasionally for watches. In the latter it has been rendered unnecessary by the introduction of the going-barrel by Swiss watchmakers, who formed teeth on the edge of the mainspring barrel to drive the train of wheels. This kind of drum is called "going" because it drives the watch during the operation of winding, which is performed by rotating the drum arbor to which the inner end of the spring is attached. A ratchet prevents the arbor from being turned backwards by the spring. The adoption of the going-barrel has been made satisfactory by the improvements in the various escapement actions.

THE ESCAPEMENT.

The spring or weight transmits its power through a train of cogs to the escapement, or device for regulating the rate at which the wheels are to revolve. In clocks a pendulum is generally used as the controlling agent. Galileo, when a student at Pisa, noticed that certain hanging lamps in the cathedral there swung on their cords at an equal rate; and on investigation he discovered the principle that the shorter a pendulum is the more quickly will it swing to and fro. As has already been observed, Huygens first applied the principle to the governing of clocks. In Fig. 202 we have a simple representation of the "dead-beat" escapement commonly used in clocks. The escape-wheel is mounted on the shaft of the last cog of the driving train, the pallet on a spindle from which depends a split arm embracing the rod and the pendulum. We must be careful to note that the pendulum controls motion only; it does not cause movement.

The escape-wheel revolves in a clockwise direction. The two pallets a and b are so designed that only one can rest on the teeth at one time. In the sketch the sloping end of b has just been forced upwards by the pressure of a tooth. This swings the pallet and the pendulum. The momentum of the latter causes a to descend, and at the instant when b clears its tooth a catches and holds another. The left-hand side of a, called the locking-face, is part of a circle, so that the escape-wheel is held motionless as long as it touches a: hence the term, "dead beat"—that is, brought to a dead stop. As the pendulum swings back, to the left, under the influence of gravity, a is raised and frees the tooth. The wheel jerks round, and another tooth is caught by the locking-face of b. Again the pendulum swings to the right, and the sloping end of b is pushed up once more, giving the pendulum fresh impetus. This process repeats itself as long as the driving power lasts—for weeks, months, or years, as the case may be, and the mechanism continues to be in good working order.

COMPENSATING PENDULUMS.

Metal expands when heated; therefore a steel pendulum which is of the exact length to govern a clock correctly at a temperature of 60° would become too long at 80°, and slow the clock, and too short at 40°, and cause it to gain. In common clocks the pendulum rod is often made of wood, which maintains an almost constant length at all ordinary temperatures. But for very accurate clocks something more efficient is required. Graham, the partner of Thomas Tompion, took advantage of the fact that different kinds of metal have different ratios of expansion to produce a self-compensating pendulum on the principle illustrated by Fig. 203. He used steel for the rod, and formed the bob, or weighted end, of a glass jar containing mercury held in a stirrup; the mercury being of such a height that, as the pendulum rod lengthened with a rise of temperature, the mercury expanded upwards sufficiently to keep the distance between the point of suspension and the centre of gravity of the bob always the same. With a fall of temperature the rod shortened, while the mercury sank in the jar. This device has not been improved upon, and is still used in observatories and other places where timekeepers of extreme precision are required. The milled nut S in Fig. 203 is fitted at the end of the pendulum rod to permit the exact adjustment of the pendulum's length.

For watches, chronometers, and small clocks

THE SPRING BALANCE

takes the place of the pendulum. We still have an escape-wheel with teeth of a suitable shape to give impulses to the controlling agent. There are two forms of spring escapement, but as both employ a hairspring and balance-wheel we will glance at these before going further.

The hairspring is made of very fine steel ribbon, tempered to extreme elasticity, and shaped to a spiral. The inner end is attached to the arbor of the balance-wheel, the outer end to a stud projecting from the plate of the watch. When the balance-wheel, impelled by the escapement, rotates, it winds up the spring. The energy thus stored helps the wheel to revolve the other way during the locking of a tooth of the escape-wheel. The time occupied by the winding and the unwinding depends upon the length of the spring. The strength of the impulse makes no difference. A strong impulse causes the spring to coil itself up more than a weak impulse would; but inasmuch as more energy is stored the process of unwinding is hastened. To put the matter very simply—a strong impulse moves the balance-wheel further, but rotates it quickly; a weak impulse moves it a shorter distance, but rotates it slowly. In fact, the principle of the pendulum is also that of the hairspring; and the duration of a vibration depends on the length of the rod in the one case, and of the spring in the other.

Motion is transmitted to the balance by one of two methods. Either (1) directly, by a cylinder escapement; or (2) indirectly, through a lever.

THE CYLINDER ESCAPEMENT

is seen in Fig. 204. The escape-wheel has sharp teeth set on stalks. (One tooth is removed to show the stalk.) The balance-wheel is mounted on a small steel cylinder, with part of the circumference cut away at the level of the teeth, so that if seen from above it would appear like a in our illustration. A tooth is just beginning to shove its point under the nearer edge of the opening. As it is forced forwards, b is revolved in a clockwise direction, winding up the hairspring. When the tooth has passed the nearer edge it flies forward, striking the inside of the further wall of the cylinder, which holds it while the spring uncoils. The tooth now pushes its way past the other edge, accelerating the unwinding, and, as it escapes, the next tooth jumps forward and is arrested by the outside of the cylinder. The balance now reverses its motion, is helped by the tooth, is wound up, locks the tooth, and so on.

THE LEVER ESCAPEMENT

is somewhat more complicated. The escape-wheel teeth are locked and unlocked by the pallets P P projecting from a lever which moves on a pivot (Fig. 205). The end of the lever is forked, and has a square notch in it. On the arbor of the balance-wheel is a roller, or plate, R, which carries a small pin, I. Two pins, B B, projecting from the plate of the watch prevent the lever moving too far. We must further notice the little pin C on the lever, and a notch in the edge of the roller.

In the illustration a tooth has just passed under the "impulse face" b of P. The lever has been moved upwards at the right end; and its forked end has given an impulse to R, and through it to the balance-wheel. The spring winds up. The pin C prevents the lever dropping, because it no longer has the notch opposite to it, but presses on the circumference of R. As the spring unwinds it strikes the lever at the moment when the notch and C are opposite. The lever is knocked downwards, and the tooth, which had been arrested by the locking-face a of pallet P, now presses on the impulse face b, forcing the left end of the lever up. The impulse pin I receives a blow, assisting the unwinding of the spring, and C again locks the lever. The same thing is repeated in alternate directions over and over again.

COMPENSATING BALANCE-WHEELS.

The watchmaker has had to overcome the same difficulty as the clockmaker with regard to the expansion of the metal in the controlling agent. When a metal wheel is heated its spokes lengthen, and the rim recedes from the centre. Now, let us suppose that we have two rods of equal weight, one three feet long, the other six feet long. To an end of each we fasten a 2-lb. weight. We shall find it much easier to wave the shorter rod backwards and forwards quickly than the other. Why? Because the weight of the longer rod has more leverage over the hand than has that of the shorter rod. Similarly, if, while the mass of the rim of a wheel remains constant, the length of the spokes varies, the effort needed to rotate the wheel to and fro at a constant rate must vary also. Graham got over the difficulty with a rod by means of the compensating pendulum. Thomas Earnshaw mastered it in wheels by means of the compensating balance, using the same principle—namely, the unequal expansion of different metals. Any one who owns a compensated watch will see, on stopping the tiny fly-wheel, that it has two spokes (Fig. 206), each carrying an almost complete semicircle of rim attached to it. A close examination shows that the rim is compounded of an outer strip of brass welded to an inner lining of steel. The brass element expands more with heat and contracts more with cold than steel; so that when the spokes become elongated by a rise of temperature, the pieces bend inwards at their free ends (Fig. 207); if the temperature falls, the spokes are shortened, and the rim pieces bend outwards (Fig. 208). This ingenious contrivance keeps the leverage of the rim constant within very fine limits. The screws S S are inserted in the rim to balance it correctly, and very fine adjustment is made by means of the four tiny weights W W. In ships' chronometers, the rim pieces are sub-compensated towards their free ends to counteract slight errors in the primary compensation. So delicate is the compensation that a daily loss or gain of only half a second is often the limit of error.

KEYLESS WINDING MECHANISM FOR WATCHES.

The inconvenience attaching to a key-wound watch caused the Swiss manufacturers to put on the market, in 1851, watches which dispensed with a separate key. Those of our readers who carry keyless watches will be interested to learn how the winding and setting of the hands is effected by the little serrated knob enclosed inside the pendant ring.

There are two forms of "going-barrel" keyless mechanism—(1) The rocking bar; (2) the shifting sleeve. The rocking bar device is shown in Figs. 209, 210. The milled head M turns a cog, G, which is always in gear with a cog, F. This cog gears with two others, A and B, mounted at each end of the rocker R, which moves on pivot S. A spring, S P, attached to the watch plate presses against a small stud on the rocking bar, and keeps A normally in gear with C, mounted on the arbor of the mainspring.

To wind the watch, M is turned so as to give F an anti-clockwise motion. The teeth of F now press A downwards and keep it in gear with C while the winding is done. A spring click (marked solid black) prevents the spring uncoiling (Fig. 209). If F is turned in a clockwise direction it lifts A and prevents it biting the teeth of C, and no strain is thrown on C.

To set the hands, the little push-piece P is pressed inwards by the thumb (Fig. 210) so as to depress the right-hand end of R and bring B into gear with D, which in turn moves E, mounted on the end of the minute-hand shaft. The hands can now be moved in either direction by turning M. On releasing the push-piece the winding-wheels engage again.

The shifting sleeve mechanism has a bevel pinion in the place of G (Fig. 209) gearing with the mainspring cog. The shaft of the knob M is round where it passes through the bevel and can turn freely inside it, but is square below. On the square part is mounted a little sliding clutch with teeth on the top corresponding with the other teeth on the under side of the bevel-wheel, and teeth similar to those of G (Fig. 209) at the end. The clutch has a groove cut in the circumference, and in this lies the end of a spring lever which can be depressed by the push-piece. The mechanism much resembles on a small scale the motor car changing gear (Fig. 49). Normally, the clutch is pushed up the square part of the knob shaft by the spring so as to engage with the bevel and the winding-wheels. On depressing the clutch by means of the push-piece it gears with the minute-hand pinion, and lets go of the bevel.

In one form of this mechanism the push-piece is dispensed with, and the minute-wheel pinion is engaged by pulling the knob upwards.

THE HOUR-HAND TRAIN.

The teeth of the mainspring drum gear with a cog on the minute-hand shaft, which also carries one of the cogs of the escapement train. The shaft is permitted by the escapement to revolve once an hour. Fig. 211 shows diagrammatically how this is managed. The hour-hand shaft A (solid black) can be moved round inside the cog B, driven by the mainspring drum. It carries a cog, C. This gears with a cog, D, having three times as many teeth. The cog E, united to D, drives cog F, having four times as many teeth as E. To F is attached the collar G of the hour-hand. F and G revolve outside the minute-hand shaft. On turning A, C turns D and E, E turns F and the hour-hand, which revolves ⅓ of ¼ = 1⁄12 as fast as A.

LOCKS.

On these unfortunately necessary mechanisms a great deal of ingenuity has been expended. With the advance of luxury and the increased worship of wealth, it becomes more and more necessary to guard one's belongings against the less scrupulous members of society.

The simplest form of lock, such as is found in desks and very cheap articles, works on the principle shown in Fig. 212. The bolt is split at the rear, and the upper part bent upwards to form a spring. The under edge has two notches cut in it, separated by a curved excrescence. The key merely presses the bolt upwards against the spring, until the notch, engaging with the frame, moves it backwards or forwards until the spring drives the tail down into the other notch. This primitive device affords, of course, very little security. An advance is seen in the

TUMBLER LOCK.

The bolt now can move only in a horizontal direction. It has an opening cut in it with two notches (Figs. 213, 214). Behind the bolt lies the tumbler T (indicated by the dotted line), pivoted at the angle on a pin. From the face of the tumbler a stud, S, projects through the hole in the bolt. This stud is forced into one or other of the notches by the spring, S, which presses on the tail of the tumbler.

In Fig. 213 the key is about to actuate the locking mechanism. The next diagram (Fig. 214) shows how the key, as it enters the notch on the lower side of the bolt to move it along, also raises the tumbler stud clear of the projection between the two notches. By the time that the bolt has been fully "shot," the key leaves the under notch and allows the tumbler stud to fall into the rear locking-notch.

A lock of this type also can be picked very easily, as the picker has merely to lift the tumbler and move the bolt along. Barron's lock, patented in 1778, had two tumblers and two studs; and the opening in the bolt had notches at the top as well as at the bottom (Fig. 215). This made it necessary for both tumblers to be raised simultaneously to exactly the right height. If either was not lifted sufficiently, a stud could not clear its bottom notch; if either rose too far, it engaged an upper notch. The chances therefore were greatly against a wrong key turning the lock.

THE CHUBB LOCK

is an amplification of this principle. It usually has several tumblers of the shape shown in Fig. 216. The lock stud in these locks projects from the bolt itself, and the openings, or "gates," through which the stud must pass as the lock moves, are cut in the tumblers. It will be noticed that the forward notch of the tumbler has square serrations in the edges. These engage with similar serrations in the bolt stud and make it impossible to raise the tumbler if the bolt begins to move too soon when a wrong key is inserted.

Fig. 217 is a Chubb key with eight steps. That nearest the head (8) operates a circular revolving curtain, which prevents the introduction of picking tools when a key is inserted and partly turned, as the key slot in the curtain is no longer opposite that in the lock. Step 1 moves the bolt.

In order to shoot the bolt the height of the key steps must be so proportioned to the depth of their tumblers that all the gates in the tumblers are simultaneously raised to the right level for the stud to pass through them, as in Fig. 218. Here you will observe that the tumbler D on the extreme right (lifted by step 2 of the key) has a stud, D S, projecting from it over the other tumblers. This is called the detector tumbler. If a false key or picking tool is inserted it is certain to raise one of the tumblers too far. The detector is then over-lifted by the stud D S, and a spring catch falls into a notch at the rear. It is now impossible to pick the lock, as the detector can be released only by the right key shooting the bolt a little further in the locking direction, when a projection on the rear of the bolt lifts the catch and allows the tumbler to fall. The detector also shows that the lock has been tampered with, since even the right key cannot move the bolt until the overlocking has been performed.

Each tumbler step of a large Chubb key can be given one of thirty different heights; the bolt step one of twenty. By merely transposing the order of the steps in a six-step key it is possible to get 720 different combinations. By diminishing or increasing the heights the possible combinations may be raised to the enormous total of 7,776,000!

THE YALE LOCK,

which comes from America, works on a quite different system. Its most noticeable feature is that it permits the use of a very small key, though the number of combinations possible is still enormous (several millions). In our illustrations (Figs. 219, 220, 221) we show the mechanism controlling the turning of the key. The keyhole is a narrow twisted slot in the face of a cylinder, G (Fig. 219), which revolves inside a larger fixed cylinder, F. As the key is pushed in, the notches in its upper edge raise up the pins A, B, C, D, E, until their tops exactly reach the surface of G, which can now be revolved by the key in Fig. 220, and work the bolt through the medium of the arm H. (The bolt itself is not shown.) If a wrong key is inserted, either some of the lower pins will project upwards into the fixed cylinder F (see Fig. 221), or some of the pins in F will sink into G. It is then impossible to turn the key.

There are other well-known locks, such as those invented by Bramah and Hobbs. But as these do not lend themselves readily to illustration no detailed account can be given. We might, however, notice the time lock, which is set to a certain hour, and can be opened by the right key or a number of keys in combination only when that hour is reached. Another very interesting device is the automatic combination lock. This may have twenty or more keys, any one of which can lock it; but the same one must be used to unlock it, as the key automatically sets the mechanism in favour of itself. With such a lock it would be possible to have a different key for every day in the month; and if any one key got into wrong hands it would be useless unless it happened to be the one which last locked the lock.

THE CYCLE.

There are a few features of this useful and in some ways wonderful contrivance which should be noticed. First,

THE GEARING OF A CYCLE.

To a good many people the expression "geared to 70 inches," or 65, or 80, as the case may be, conveys nothing except the fact that the higher the gear the faster one ought to be able to travel. Let us therefore examine the meaning of such a phrase before going farther.

The safety cycle is always "geared up"—that is, one turn of the pedals will turn the rear wheel more than once. To get the exact ratio of turning speed we count the teeth on the big chain-wheel, and the teeth on the small chain-wheel attached to the hub of the rear wheel, and divide the former by the latter. To take an example:—The teeth are 75 and 30 in number respectively; the ratio of speed therefore = 75⁄30 = 5⁄2 = 2½. One turn of the pedal turns the rear wheel 2½ times. The gear of the cycle is calculated by multiplying this result by the diameter of the rear wheel in inches. Thus a 28-inch wheel would in this case give a gear of 2½ × 28 = 70 inches.

One turn of the pedals on a machine of this gear would propel the rider as far as if he were on a high "ordinary" with the pedals attached directly to a wheel 70 inches in diameter. The gearing is raised or lowered by altering the number ratio of the teeth on the two chain-wheels. If for the 30-tooth wheel we substituted one of 25 teeth the gearing would be—

75⁄25 × 28 inches = 84 inches.

A handy formula to remember is, gearing = T/t × D, where T = teeth on large chain-wheel; t = teeth on small chain-wheel; and D = diameter of driving-wheel in inches.

Two of the most important improvements recently added to the cycle are—(1) The free wheel; (2) the change-speed gear.

THE FREE WHEEL

is a device for enabling the driving-wheel to overrun the pedals when the rider ceases pedalling; it renders the driving-wheel "free" of the driving gear. It is a ratchet specially suited for this kind of work. From among the many patterns now marketed we select the Micrometer free-wheel hub (Fig. 222), which is extremely simple. The ratchet-wheel R is attached to the hub of the driving-wheel. The small chain-wheel (or "chain-ring," as it is often called) turns outside this, on a number of balls running in a groove chased in the neck of the ratchet. Between these two parts are the pawls, of half-moon shape. The driving-wheel is assumed to be on the further side of the ratchet. To propel the cycle the chain-ring is turned in a clockwise direction. Three out of the six pawls at once engage with notches in the ratchet, and are held tightly in place by the pressure of the chain-ring on their rear ends. The other three are in a midway position.

When the rider ceases to pedal, the chain-ring becomes stationary, but the ratchet continues to revolve. The pawls offer no resistance to the ratchet teeth, which push them up into the semicircular recesses in the chain-ring. Each one rises as it passes over a tooth. It is obvious that driving power cannot be transmitted again to the road wheel until the chain-wheel is turned fast enough to overtake the ratchet.

THE CHANGE-SPEED GEAR.

A gain in speed means a loss in power, and vice versâ. By gearing-up a cycle we are able to make the driving-wheel revolve faster than the pedals, but at the expense of control over the driving-wheel. A high-geared cycle is fast on the level, but a bad hill-climber. The low-geared machine shows to disadvantage on the flat, but is a good hill-climber. Similarly, the express engine must have large driving-wheels, the goods engine small driving-wheels, to perform their special functions properly.

In order to travel fast over level country, and yet be able to mount hills without undue exertion, we must be able to do what the motorist does—change gear. Two-speed and three-speed gears are now very commonly fitted to cycles. They all work on the same principle, that of the epicyclic train of cog-wheels, the mechanisms being so devised that the hub turns more slowly than, at the same speed as, or faster than the small chain-wheel, according to the wish of the rider.

We do not propose to do more here than explain the principle of the epicyclic train, which means "a wheel on (or running round) a wheel." Lay a footrule on the table and roll a cylinder along it by the aid of a second rule, parallel to the first, but resting on the cylinder. It will be found that, while the cylinder advances six inches, the upper rule advances twice that distance. In the absence of friction the work done by the agent moving the upper rule is equal to that done in overcoming the force which opposes the forward motion of the cylinder; and as the distance through which the cylinder advances is only half that through which the upper rule advances, it follows that the force which must act on the upper rule is only half as great as that overcome in moving the cylinder. The carter makes use of this principle when he puts his hand to the top of a wheel to help his cart over an obstacle.

Now see how this principle is applied to the change-speed gear. The lower rule is replaced by a cog-wheel, C (Fig. 223); the cylinder by a cog, B, running round it; and the upper rule by a ring, A, with internal teeth. We may suppose that A is the chain-ring, B a cog mounted on a pin projecting from the hub, and C a cog attached to the fixed axle. It is evident that B will not move so fast round C as A does. The amount by which A will get ahead of B can be calculated easily. We begin with the wheels in the position shown in Fig. 223. A point, I, on A is exactly over the topmost point of C. For the sake of convenience we will first assume that instead of B running round C, B is revolved on its axis for one complete revolution in a clockwise direction, and that A and C move as in Fig. 224. If B has 10 teeth, C 30, and A 40, A will have been moved 10⁄40 = ¼ of a revolution in a clockwise direction, and C 10⁄30 = ⅓ of a revolution in an anti-clockwise direction.

Now, coming back to what actually does happen, we shall be able to understand how far A rotates round C relatively to the motion of B, when C is fixed and B rolls (Fig. 225). B advances ⅓ of distance round C; A advances ⅓ + ¼ = 7⁄12 of distance round B. The fractions, if reduced to a common denominator, are as 4:7, and this is equivalent to 40 (number of teeth on A): 40 + 30 (teeth on A + teeth on C.)

To leave the reader with a very clear idea we will summarize the matter thus:—If T = number of teeth on A, t = number of teeth on C, then movement of A: movement of B:: T + t: T.

Here is a two-speed hub. Let us count the teeth. The chain-ring (= A) has 64 internal teeth, and the central cog (= C) on the axle has 16 teeth. There are four cogs (= B) equally spaced, running on pins projecting from the hub-shell between A and C. How much faster than B does A run round C? Apply the formula:—Motion of A: motion of B:: 64 + 16: 64. That is, while A revolves once, B and the hub and the driving-wheel will revolve only 64⁄80 = ⅘ of a turn. To use scientific language, B revolves 20 per cent. slower than A.

This is the gearing we use for hill-climbing. On the level we want the driving-wheel to turn as fast as, or faster than, the chain-ring. To make it turn at the same rate, both A and C must revolve together. In one well-known gear this is effected by sliding C along the spindle of the wheel till it disengages itself from the spindle, and one end locks with the plate which carries A. Since B is now being pulled round at the bottom as well as the top, it cannot rotate on its own axis any longer, and the whole train revolves solidly—that is, while A turns through a circle B does the same.

To get an increase of gearing, matters must be so arranged that the drive is transmitted from the chain-wheel to B, and from A to the hub. While B describes a circle, A and the driving-wheel turn through a circle and a part of a circle—that is, the driving-wheel revolves faster than the hub. Given the same number of teeth as before, the proportional rates will be A = 80, B = 64, so that the gear rises 25 per cent.

By means of proper mechanism the power is transmitted in a three-speed gear either (1) from chain-wheel to A, A to B, B to wheel = low gear; or (2) from chain-wheel to A and C simultaneously = solid, normal, or middle gear; or (3) from chain-wheel to B, B to A, A to wheel = high gear. In two-speed gears either 1 or 3 is omitted.

AGRICULTURAL MACHINES.

THE THRESHING-MACHINE.

Bread would not be so cheap as it is were the flail still the only means of separating the grain from the straw. What the cream separator has done for the dairy industry (p. 384), the threshing-machine has done for agriculture. A page or two ought therefore to be spared for this useful invention.

In Fig. 226 a very complete fore-and-aft section of the machine is given. After the bands of the sheaves have been cut, the latter are fed into the mouth of the drum A by the feeder, who stands in the feeding-box on the top of the machine. The drum revolves at a very high velocity, and is fitted with fluted beaters which act against a steel concave, or breastwork, B, the grain being threshed out of the straw in passing between the two. The breastwork is provided with open wires, through which most of the threshed grain, cavings (short straws), and chaff passes on to a sloping board. The straw is flung forward on to the shakers C, which gradually move the straw towards the open end and throw it off. Any grain, etc., that has escaped the drum falls through the shakers on to D, and works backwards to the caving riddles, or moving sieves, E. The main blower, by means of a revolving fan, N, sends air along the channel X upwards through these riddles, blowing the short straws away to the left. The grain, husks, and dust fall through E on to G, over the end of which they fall on to the chaff riddle, H. A second column of air from the blower drives the chaff away. The heavy grain, seeds, dust, etc., fall on to I, J, and K in turn, and are shaken until only the grain remains to pass along L to the elevator bottom, M. An endless band with cups attached to it scoops up the grain, carries it aloft, and shoots it into hopper P. It then goes through the shakers Q, R, is dusted by the back end blower, S, and slides down T into the open end of the rotary screen-drum U, which is mounted on the slope, so that as it turns the grain travels gradually along it. The first half of the screen has wires set closely together. All the small grain that falls through this, called "thirds," passes into a hopper, and is collected in a sack attached to the hopper mouth. The "seconds" fall through the second half of the drum, more widely spaced, into their sack; and the "firsts" fall out of the end and through a third spout.

MOWING-MACHINES.

The ordinary lawn—mower employs a revolving reel, built up of spirally-arranged knives, the edges of which pass very close to a sharp plate projecting from the frame of the mower. Each blade, as it turns, works along the plate, giving a shearing cut to any grass that may be caught between the two cutting edges. The action is that of a pair of scissors (Fig. 227), one blade representing the fixed, the other the moving knife. If you place a cylinder of wood in the scissors it will be driven forward by the closing of the blades, and be marked by them as it passes along the edges. The same thing happens with grass, which is so soft that it is cut right through.

HAY-CUTTER.

The hay-cutter is another adaptation of the same principle. A cutter-bar is pulled rapidly backwards and forwards in a frame which runs a few inches above the ground by a crank driven by the wheels through gearing. To the front edge of the bar are attached by one side a number of triangular knives. The frame carries an equal number of spikes pointing forward horizontally. Through slots in these the cutter-bar works, and its knives give a drawing cut to grass caught between them and the sides of the spikes.

SOME NATURAL PHENOMENA.

WHY SUN-HEAT VARIES IN INTENSITY.

The more squarely parallel heat-rays strike a surface the greater will be the number that can affect that surface. This is evident from Figs. 228, 229, where A B is an equal distance in both cases. The nearer the sun is to the horizon, the more obliquely do its rays strike the earth. Hence midday is necessarily warmer than the evening, and the tropics, where the sun stands overhead, are hotter than the temperate zones, where, even in summer at midday, the rays fall more or less on the slant.

The atmospheric envelope which encompasses the earth tends to increase the effect of obliquity, since a slanting ray has to travel further through it and is robbed of more heat than a vertical ray.

THE TIDES.

All bodies have an attraction for one another. The earth attracts the moon, and the moon attracts the earth. Now, though the effect of this attraction is not visible as regards the solid part of the globe, it is strongly manifested by the water which covers a large portion of the earth's surface. The moon attracts the water most powerfully at two points, that nearest to it and that furthest away from it; as shown on an exaggerated scale in Fig. 230. Since the earth and the water revolve as one mass daily on their axis, every point on the circumference would be daily nearest to and furthest from the moon at regular intervals, and wherever there is ocean there would be two tides in that period, were the moon stationary as regards the earth. (It should be clearly understood that the tides are not great currents, but mere thickenings of the watery envelope. The inrush of the tide is due to the temporary rise of level.)

WHY HIGH TIDE VARIES DAILY.

The moon travels round the earth once in twenty-eight days. In Fig. 231 the point a is nearest the moon at, say, twelve noon. At the end of twenty-four hours it will have arrived at the same position by the compass, but yet not be nearest to the moon, which has in that period moved on 1⁄28th of a revolution round the earth. Consequently high tide will not occur till a has reached position b and overtaken the moon, as it were, which takes about an hour on the average. This explains why high tide occurs at intervals of more than twelve hours.

NEAP TIDES AND SPRING TIDES.

The sun, as well as the moon, attracts the ocean, but with less power, owing to its being so much further away. At certain periods of the month, sun, earth, and moon are all in line. Sun and moon then pull together, and we get the highest, or spring tides (Fig. 232). When sun and moon pull at right angles to one another—namely, at the first and third quarters—the excrescence caused by the moon is flattened (Fig. 233), and we get the lowest, or neap tides.