Page:Philosophical Transactions - Volume 096.djvu/30

18 conceive also a species of accumulated force residing in a moving body, which is capable of resisting pressure during a time that is proportional to its momentum or quantitas motus.

If it be of any real utility to give the name of force to this complex idea of vis motrix extended through time, as well as that of momentum to its effects when unresisted, it would be requisite to distinguish this force always by some such appellation as momental force; for it is to be apprehended that for want of this distinction many writers themselves, and it is certain that many readers of diquisitions on this subject have confounded and compared together vis motrix, momentum, and vis mechanica: quantities, that are all of them totally dissimilar, and bear no more comparison to each other, than lines to surfaces, or surfaces to solids.

In practical mechanics, however, it is at least very rarely that the momentum of bodies is in any degree an object of consideration: the strength of machinery being in every case to be adapted to the quantitas motrix, and the extent and value of the effect to be produced depending upon the quantitas mechanica of the force applied, or in other words to the space through which a given vis motrix is exerted.

The comparative velocities given by different quantities of mechanic force to bodies of equal or unequal magnitude have been so distinctly treated of by, in a series of most direct experiments, that it would be a needless waste of time to reconsider them in this place. So also, on the contrary, the quantities of extended mechanic effect producible by bodies moving with different quantities of impetus have been as clearly traced by the same accurate experimentalist.

But there is one view, in which the comparative forces of