Note to Chapter III. How to read Differentials

It will never do to fall into the schoolboy error of thinking that means d times x, for d is not a factor-it means "an element of" or "a bit of" whatever follows. One reads thus: "dee-eks."

In case the reader has no one to guide him in such matters it may here be simply said that one reads differential coefficients in the following way. The differential coefficient

So also is read "dee-you by dee-tee." Second differential coefficients will be met with later on. They are like this: and it means that the operation of differentiating y with respect to x has been (or has to be) performed twice over.

Another way of indicating that a function has been differentiated is by putting an accent to the symbol of the function. Thus if , which means that y is some unspecified function of x (see p. 13), we may write instead of . Similarly, will mean that the original function has been differentiated twice over with respect to x.