Chap. i. PHILOSOPHY. 47

with half the velocity, which the body first in motion had:that is, the body first moved will have lost half its motion,and the other will have gained exactly as much. This ruleis just, provided the bodies keep contiguous after meeting; asthey would always do, if it were not for a certain cause thatoften intervenes, and which must now be explained. Bodiesupon striking against each other, suffer an alteration in theirfigure, having their parts pressed inwards by the stroke,whichfor the most part recoil again afterwards, the bodies endea-vouring to recover their former shape. This power, wherebybodies are inabled to regain their first figure, is usually calledtheir elasticity, and when it acts, it forces the bodies fromeach other, and causes them to separate. Now the effect ofthis elasticity in the present case is such, that if the bodies areperfectly elastic, so as to recoil with as great a force as theyare bent with, that they recover their figure in the fame spaceof time, as has been taken up in the alteration made in it bytheir compression together; then this power will separate thebodies as swiftly, as they before approached, and acting up-on both equally, upon the body first in motion contrary to.the direction in which it moves, and upon the other as muchin the direction of its motion, it will take from the first, andadd to the other equal degrees of velocity: so that the powerbeing strong enough to separate them with as great a velocity,as they approached with, the first will be quite stopt, andthat which was at rest, will receive all the motion of theother. If the bodies are elastic in a left degree, the first willnot lose all its motion, nor will the other acquire the motionof the first, but fall as much short of it, as the other retains..

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