" This, which is called the principle of virtual velocities, is one that should never be lost sight of, in mechanics.

Were it always remembered, much time, vainly expended by ingenious persons, in seeking for what is called the " perpetual motion," would be saved : since, the province of machinery is not to create momentum, but to modify its elements.

And were it possible to construct an infinitely perfect machine, we should receive from it, only that amount of motion which we imparted to it.

But the very best, we can form, consumes some power : and the amount, destroyed, is dependent, in a great degree, on its complication.

We are, to conclude, therefore, that machinery should never be used, except where it cannot be avoided ; and that, when used, it should be as simple as possible.