Sir I s a ac Newton’s Book I.

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pendulum acquires in G, is proportional to this line : that is, ifthe pendulum, after it has descended from E to G, be takenback to H, and let fall from thence, and the line H G bedrawn ; the velocity, which the pendulum shall acquire inG by its descent from H, shall bear the fame proportion tothe velocity, which it acquires in falling from E to G, as thestraight line H G bears to the straight line E G.

73. We may now proceed to those experiments upon thepercussion of bodies, which I observed above might bemade with pendulums. This expedient for examining theeffects of percussion was first proposed by our late greatarchitect Sir ChristopherWren. And it is as follows.Two balls, as A and B (in fig. yq.) either equal or une-qual, are hung by two strings from two points C and D, sothat, when the balls hang down without motion, they shalljust touch each other, and the strings be parallel. Here ifone of these balls be removed to any distance from its perpen-dicular situation, and then let fall to descend and strike a-gainst the other; by the last preceding paragraph it will beknown, with what velocity this ball shall return into its firstperpendicular situation, and consequently with what force itshall strike against the other ball; and by the height to whichthis other ball ascends after the stroke, the velocity commu-nicated to this ball will be discovered. For instance, let theball A be taken up to E, and from thence be let fall to strikeagainst B, passing over in its descent the circular arch E F.By this impulse let B fly up to G, moving through the circu-lar arch HG. Then El and GK being drawn horizontally,

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