The cases in which a force is apparently increased or diminished, as well as those in which it seems to disappear, are all found, on examination, to be illusive.  For example, the apparent increase of a man’s power by the use of a lever is really no increase at all.  It is true that, by pressing upon the outer arm with his own weight, he can cause the much greater weight of the stone to rise; but then it will rise only a very little way in comparison with the distance through which his own weight descends.  His own weight must, in fact, descend through a distance as much greater than that by which the stone ascends, as the weight of the stone is greater than his weight.  In other words, so far as the balance of the forces is concerned, the whole amount of the downward motion consists of the smaller weight descending through a greater distance, which will be equal to the whole amount of that of the larger one ascending through a smaller distance; and, to produce a preponderance, the whole amount of the downward force must be somewhat greater.  Thus the lever only gathers or concentrates force, as it were, but does not at all increase it.

It is so with all the other contrivances for managing force for the accomplishment of particular purposes.  None of them, increase the force, but only alter its form and character, with a view to its better adaptation to the purpose in view.

Nor can any force be extinguished.  When a bullet strikes against a solid wall, the force of its movement, which seems to disappear, is not lost; it is converted into heat—­the temperature of both the bullet and of that part of the wall on which it impinges being raised by the concussion.  And it is found that the amount of the heat which is thus produced is always in exact proportion to the quantity of mechanical motion which is stopped; this quantity depending on the weight of the bullet, and on the velocity with which it was moving.  And it has been ascertained, moreover, by the most careful, patient, and many times repeated experiments and calculations, that the quantity of this heat is exactly the same with that which, through the medium of steam, or by any other mode of applying it, may be made to produce the same quantity of mechanical motion that was extinguished in the bullet.  Thus the force was not destroyed, but only converted into another form.

And if we should follow out the natural effects of this heat into which the motion of the bullet was transferred, we should find it rarefying the air around the place of concussion, and thus lifting the whole mass of the atmosphere above it, and producing currents of the nature of wind, and through these producing other effects, thus going on forever; the force changing its form, but neither increasing or diminishing its quantity through a series of changes without end.

The Arrest and temporary Reservation of Force.