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TRACT 34. IN GUNNERY.

after the stroke, they may both proceed forward togetherwith a common velocity. By this means, it is obvious thatthe original velocity of the ball may be reduced in any pro-portion, or to any slow motion which may conveniently bemeasured, by making the body struck to be sufficientlylarge : for it is well known that the common velocity, withwhich’'the ball and the block of wood would move on toge-ther, after the stroke, bears to the original velocity of theball before the stroke, the same ratio which the weight ofthe ball has to that of the ball and block together. Thusthen the velocities of 1000 feet in a second, are easily reducedto those of 2 or 3 feet only: which small velocity beingmeasured by any convenient means, and the number denot-ing it increased in the ratio of the weight of the ball, to theweight of the ball and block together, the original velocityof the ball itself will thereby be obtained.

7. Now this reduced velocity is rendered easy to be mea-sured by a very simple and curious contrivance, of Mr.Robins, which is this: the block of wood, which is struckby the ball, instead of being left at liberty to move straightforward in the direction of the ball’s motion, is sus-pended, like the weight of the vibrating pendulum of aclock, by a strong iron stem, having a horizontal axis atthe top, on the ends of which it vibrates freely when struckby the ball. The consequence of this simple contrivance isevident: this large ballistic pendulum, after being struckby the ball, is penetrated by it to a small depth, and it thenswings round its axis, describing an arch, which is greateror less according to the force of the blow struck ; and fromthe magnitude of the arch described by the vibrating pen-dulum, the velocity of any point of the pendulum can beeasily computed : for, a body acquiring the same velocity byfalling from the same height, whether it descend perpendi-cularly down, or otherwise: therefore, having given thelength of the arc described by the centre of oscillation, orany other point, and its radius, the versed sine becomesknown, which is the height perpendicularly descended by