Note 3 — Chapter IV · SIMPLEST CASES.
Reading edition
N.B.—This ratio is the result of differentiating with respect to . Differentiating means finding the differential coefficient. Suppose we had some other function of , as, for example, . Then if we were told to differentiate this with respect to , we should have to find , or, what is the same thing, . On the other hand, we may have a case in which time was the independent variable (see p. 14), such as this: . Then, if we were told to differentiate it, that means we must find its differential coefficient with respect to . So that then our business would be to try to find , that is, to find .