222

'fHEOUV AND PRACTICE

TRACT 37.

as compared with tli.it of air rushing into a completevacuum, that is, while the former is not greater than the lat-ter ; for, as soon as the motion of the ball becomes equal tothat of the air, and always when greater, then the hall has tosustain the whole pressure of the atmosphere on its forepart,■without having any aid from a counter-pressure behind.

23. Thus then we see that the resistance against the ballis two-fold, (besides that arising from the unknown degreeof compression before the ball), the one arising from percus-sion, by the ball striking and displacing the particles of airin its path, and which increases continually in the duplicateproportion of the ball’s velocity; and the other from theweight of the atmosphere, increasing with that velocity, towhich, being of the nature of pressure, it is proportional;but arriving at its maximum when that is equal to or exceedsthe velocity of air iuto a vacuum, after which it is a constantquantity for all greater degrees of velocity. Those circum-stances then very well show the reason why the experimentedresistance proceeds in a ratio increasing gradually more andmore above the square of the velocity, till this exceeds 12 or14 hundred feet, the motion of air into a vacuum, and thenrather decreases again. So that it appears that the wholeestimatable resistance consists qf two parts; of which the onepart is proportional to the square of the velocity, and theother is simply as the velocity only.

24. Besides the experiments hitherto noticed, others wereat several times performed, both with guns and mortars, dis-charged with different degrees of elevation and velocities, toobserve the extent of the ranges, the times of flight, and theangles at which the shots fall to the ground; a synopsis qfsuch experiments of this kind as have been made, is containedin the following table.