Page:Proceedings of the Royal Society of London Vol 1.djvu/234

Rh idea which, the author observes, arises naturally from the daily oc- cupations of men, since any quantity of work performed is always estimated by the extent of effect resulting from their exertions. Thus it is well known that the raising of any great weight 40 feet would require four times as much labour as would be requisite to raise an equal weight 10 fcct. And if Weights so raised were suffered to fall freely, the squares of the velocities acquired would be in proportion to the quantity of labour, that is, as 4 to l g and if their forces were employed in driving piles, the effects produced would be in that same ratio.

This species of force has, by Smeaton, been aptly denominated mechanic force; and when by force of percussion is meant the quan- tity of mechanic force which a body in motion can exert, the author apprehends it cannot be controverted that the said force is in pro- portion to the magnitude of the body, and the square of its velocity joinﬂ.

But of this force Newton nowhere treats, and consequently gives no deﬁnition of it; on the contrary, in the preface to the Principia. he expressly says, that he writes " de potentiis non manualibus, sed naturalibus ;" and again, in the Scholium to the laws of motion, he says, “ Caeteruxn mechanicam tractare, non est hujus instituti."

It is also. evident, that in the third law of motion, when Newton aserts that action is equal to reaction, he means only that the mov- ing forces, or pressures opposed to each other, are necessarily cqual. Other persons, however, have interpreted the third law differently, and conceive also a species of accumulated force, which is capable of resisting a- given pressure, during a time that is proportional to the momentum, or quantitas moms.

If it be of any real utility to give the name of force to such a com- plex idea of via motmlz‘ continued for any certain time, the author re- commends that it should be always distinguished by some such ap- pellation as momentous force, as he apprehends that, for want of this distinction, both writers and readers of disquisitions upon this subject have confoundcd and compared together vis matrix, 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 sur- faces to solids.

In practical mechanics, however, it is'at least very rzu'cly that that momentum of bodies is an object of consideration; since the cxtent and value of any effect to he produced depends upon the guantitas meelzanica of the force applied, or in other words, the space through which any moving force is excrted.

Dr. Wollaston, in the next place, compares the forces of the different bodies by means which he is inclined to think have not been taken notice of by any writer on this question; and he shows, that when the whole energy of a body A is employed without kiss, in giving velocity to a second body B, the impetus which B receives is, in all cases, equal to that of A, the squares of their velocities being in the reciprocal ratio of the bodies.