IV. AIR PCJIP.
The elasticity of air having been ascertained by the ex periments of Boyle, the ingenious soon availed themselves of it in the construction of air-pumps. The principal ob jections to that which Boyle employed were, the inconve nience of alternately shutting and opening the stop-cock and valve, and the great difficulty of making the piston descend when the internal air is considerably rarefied. The first of these was surmounted by employing the air itself as a mechanical agent ; and the second by employ ing two pistons connected in such a manner that the one was depressed when the other was elevated, by which means the pressure of the external air which opposed the descent of the one favoured the ascent of the other. An improved machine of this nature is represented in (Fig. 10.) which receives the name of Hawksbee's pump, from the individual who contrived it. KR are the two piston rods which move by the revolution of the wheel \V in their respective cylinders CC. PL PL are two pistons which are fixed perpendicularly to the frame work of the instru ment ; over these is placed the transverse bar of wood PP, into which the upper extremity of the cylinders are fixed, and to which the wheel \V is attached, which is turned by the handle H. Their lower ends rest upon two circular pieces of brass, which contain each a valve covering the extremities of the horizontal tube AA. The valves were formed by covering two very small holes pierced through the brass by a slip of limber bladder which was fastened by a silken thread wound around the circumference, and embracing its extremities.
These valves are so constructed as to be distended up wards by the elasticity of the internal air, and resemble, in every particular, the two valves which are at the end of the piston rods. The tube AA is connected with the pipe T, which passes up through a hole in the board BB, over which the receiver is placed. This tube is usually fitted with a stop-cock, which is shut after a vacuum has been made, in order to prevent any air which may enter at AA, or any other part of the pimp, from getting access to the receiver. The manner in which the air is exhausted, is
obvious from the construction of the machine ; when the handle is turned, one of the pistons is raised and the other depressed. The air contained in the cylinder of the as cending piston is dilated, for no external air can enter, as its pressure shuts the valve of the piston rod by compress ing the bladder upon the puncture; hence its elasticity becomes less than that of the air contained within the re ceiver and tube AA, which, of consequence, opens the lower valve and expands itself into the cylinder until the density of both become equal. On turning the handle in a contrary direction, the elevated piston is made to de scend, and by compressing the air beneath it, which can not force itself back into the receiver, owing to its increas ed elasticity, it at length exceeds in density the external air, and, opening the piston valve, makes its escape. The same process is repeated until any required degree of ex haustion is obtained.
From the manner in which the air is exhausted a formu la may be given, by which the degree of exhaustion pro duced after a given number of strokes may be found. For let V be the capacity of the receiver, v that of the barrel, and the density of the air. Then, as on raising the pis ton for the first time, the volume is exhausted, V is the portion of air which remains, and as this expands and tills the space VA-v, ei`NT , is its density.
By' raising the piston a second time, the volume v of the V density - V + v is exhausted, so that there will remain as before the volume V; and, as this air is dilated so as to fill V + -v, its density becomes (V + Hence the density of the air which remains after the nth stroke is --• (V + el` Vn v v 12 v)n V — — -- is an expression for the degree of rarefaction produced.