[It is absolutely essential that the reader should have clear notions on these points, for there is widespread confusion and error as to the meaning even of so simple and elementary a term as ‘_force_.’ He will often find it used indifferently in either of two senses which have no connection whatever with one another; and unless he completely gets over this abuse of language he need not hope to be able to follow the present portion of our preliminary argument. Force proper is a pull, push, weight, pressure, etc., and can be measured, in the vernacular of engineers, as equivalent to so many pounds weight; but the unjustifiable use of the word applies it to _work done by a force_, so many pounds raised so many feet, _i.e. force overcome through a space_. Two such things are of different kinds, and cannot possibly be compared together. They differ in fact in precisely the same way as length or breadth differs from superficial area, _i.e._ as a linear foot differs from a square foot! And the modern abuse of the word is more outrageous, alike to science and to common sense, than would be the attempt to assign the height of a mountain in acres! For the absurdity does not end even here. We have, as yet, absolutely no proof whatever that force proper has objective existence. In all probability there is no such _thing_ as force (which is suggested to us by the impressions of our _muscular_ sense), any more than there is such a _thing_ as Sound, or Light, which are mere names for physical impressions produced upon special nerves by the energy of undulatory motions of certain media. The term, however, is a very convenient one for the rate of transference or transformation of energy per unit of length in a given direction.]