66 can in fome fort avoid the preffure of the atmosphere on its hinder parts. It may perhaps "confirm this conjecture to obferve, that if a .66 bullet, moving with the velocity of found, does really leave a vacuum behind it, the preffure of "the atmosphere on its fore part is a force about "three times as great as its resistance, computed "by the laws observed for flow motions.*" A fhot, befides its being drawn downwards from the line of direction, is fometimes deflected fideway; which takes place when the shot by rubbing against one side of the cavity of the piece, acquires a rotatory motion round its axis, and proceeds through the air with that motion; for in that cafe the fide of the shot, which in its courfe through the air turns forwards, meets with greater resistance than the oppofite fide, whose motion coincides with that of the air. It is eafy to conceive that when the axis of rotation happens to be parallel to the horizon, then the rotation will contribute to the fhot's deflection, not fideways, but upwards or downwards. We shall lastly observe as a strong instance of * Robins's Effays on Gunnery.-The reader may derive confiderable information from Dr. Hutton's Paper on the Force of fired Gunpowder, and the initial Velocities of Cannon Ball, &c. in the 68th vol. of the Phil. Trans. And from the Chev. de Borde's Memoirs in the Hift. of the Acad. of Scienc. for 1769. the the difficulty which attends the practical management of projectiles, that even with the very fame piece of ordnance, like fhots, equal weights of the fame fort of gunpowder, and the fame angle of elevation, the places on which the shots ftrike the horizontal plane frequently differ by several yards from each other. END OF THE FIRST VOLUME, Luke Hanfard, Printer, |