The only changes possible are changes of direction, changes in the relation of intensity to quantity, and changes of form.
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.