TRACT 34.

IN GUNNER?.

321

other in a horizontal position. Then, as it lay, the distancewas measured from the middle of the axis to the part whichrested on the edge of the prism, or the place of the centreof gravity, which is the distance g of that centre belowthe axis.

19. The other method is this: The ends of the axis beingsupported on fixed up-rights, and a chord fas-tened to the lower endof the block, or of thegun-frame, and passedover a pulley at p, dif-ferent weights w werefastened to the other endof it, till the body wasbrought to a horizontalposition. Then, taking also the whole weight of the body, and.its length from the axis to the bottom, where the chordwas fixed, the place of the centre of gravity is found by thisproportion :

As p the weight of the pendulum i

is to it; the appended weight::

so is d the whole length from the axis to the chord:

to — the distance from the axis to the centre of gravity.

Either of these two methods gave the place of the centreof gravity sufficiently exact ; but the agreement of theresults of both of them was still more satisfactory.

20. To find the centre of oscillation, the ballistic pen-dulum, or the gun, was hung up by its axis in its place,and then made to vibrate in small arcs, for 1 minute, or 2,or 5, or 10 minutes ; the more the better ; as determinedeither by a half seconds pendulum, or a stop watch, or apeculiar time-piece, measuring the time to 40th parts ofa second ; and the number of vibrations performed in thattime carefully counted. Having thus obtained the timeanswering to a certain number of vibrations, the centre ofoscillation is easily found; for, if n denote the number of

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