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Sir Isaac Newton’s Book I.

they remain contiguous after the stroke; but that if they areelastic, they separate, and that the degree of their elasticitydetermines the proportion between the celerity whetewiththey separate, and the celerity wherewith they meet. Nowour author found, that the degree of elasticity appeared inthe fame bodies always the fame, with whatever degree offorce they struck ; that is, the celerity wherewith they se-parated, always bore the seme proportion to the celeritywherewith they met: so that the elastic power in all the bo-dies, he made trial upon, exerted it self in one constant pro-portion to the compresting force. Our author made trialwith balls of wool bound up very compact, and found thecelerity with which they receded, to bear about the propor-tion of y to 9 to the celerity wherewith they met; and insteel he found nearly the seme proportion; in cork the elasti-city was something less; but in glass much greater; for thecelerity, wherewith balls of that material separated after per-cussion, he sound to bear the proportion of I f to 1 6 to thecelerity wherewith they met \

7 y. I shall finish my discourse on pendulums, withthis farther observation only, that the center of oscillation isalso the center of another force. If a body be fixed to anypoint, and being put in motion turns round it; the body, ifuninterrupted by the power of gravity or any other means,will continue, perpetually to move about with the seme equa-ble motion. Now the force, with which such a body

* Princip. Philos. pag. 25.

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moves,