194

EXPERIMENTS WITH

TRACT 36.

But if the cylinder be taken, whose resistance r = l - 526 :then x — \*r = 5'72 ; which exceeds the height, 4, due tothe velocity, in the ratio of 23 to 16 nearly. And the differ-ence would be still greater, if the body were larger, and alsoif the velocity were more.

(8) . Also, if it be required to find with what velocity anyflat surface must be moved, so as to suffer a resistance justequal to the whole pressure of the atmosphere :

The resistance on the whole circle whose area being |.ofafoot, is ‘051 oz, with the velocity of 3 feet per second ; it isl of ‘051, or '0056 oz only, with a velocity of 1 foot. But2£ x 13600 x 5 = 7555-^ oz, is the whole pressure of theatmosphere. Therefore, as ^/ - 0056 : y/7556 : : 1 : 1162nearly, which is the velocity sought. Being almost equal tothe velocity with which air rushes into a vacuum.

(9) . Hence may be inferred the great resistance sufferedby military projectiles. For, in the table, it appears, that aglobe of 6 -| inches diameter, which is equal to the size of aniron ball weighing 36lb, moving with a velocity of only 16feet per second, meets with a resistance equal to the pres-sure of ■§. of an ounce weight; and therefore, computingonly according to the square of the velocity, the least re-sistance that such a ball would meet with, when moving witha velocity of 1600 feet, would be equal to the pressure of417lb, and that independent of the pressure of the atmo-sphere itself on the fore part of the ball, which would be487l'o more, as there would be no pressure from the atmo-sphere on the hinder part, in the case of so great a velocityas 1600 feet per second. So that the whole resistance wouldbe more than 900ib to such a velocity!

(10) . Having said, in the last article, that the pressure ofthe atmosphere is taken entirely off the hinder part of theball moving with a velocity of 1600 feet per second ; whichmust happen, when the ball moves faster than the particlesof air can follow, by rushing into the place quitted and leftvoid by the ball, or when the ball moves faster than the airrushes into a vacuum from the pressure of the incumbent