i66 Double-barrel Pump of new Conjlru^ion. 



Defcription of a Double-barreled Air Pump. 



^ig. 2. rcprefents a feclion of the pump, &c. where A and B are the two barrels, open 

 at top, and furnifhed with piftons, which are worked by rack and wheel in the ufual 

 manner. The piftons are alike in each barrel, and are perforated through the centre, as is 

 the folid part of the rack alfo, to admit the hollow wire a a to Aide therein. This wire is 

 made to move air tight, by means of collars of leather in the centre of the pifton. The 

 wire is hollow from end to end, and is fixed to the centre of the bottom of the barrel. 

 Near the upper end of the wire a hole is drilled through it, at right angles with that in its 

 axis i this hole is at fuch a diftance from the bottom of the barrel, that, when the pifton 

 is drawn up, the hole opens into the barrel, as is feen in the barrel B. Each pifton is 

 furniftied with a common filk valve g, opening upwards. The hole in the pifton rack, 

 through which the wire Aides, is fecured at top by the flop fcrew h. The barrels are 

 flanched on the bottom piece c c, into which the Aiding wires are fixed as above defcribed. 

 The hollow wire in the barrel B communicates at bottom with the duft d b, leading 

 through the cock c, and that in the barrel A communicates with the du£l ?_/;_/ is the 

 hole leading back to the receiver. 



The cock c is perforated, as reprefented in the figure, and has a motion of one quartet 

 of a revolution to the left, from its prefent pofition. The piftons (hould be well leathered, 

 and always have about a quarter of an inch of oil on their upper furfaces. The operation 

 of this pump is as follows : — Let the cock ftand as reprefented in the figure, in which 

 fituation it opens a communication between the receiver and the barrel B, through the 

 duft c b and Aiding wire, when the pifton is drawn up. When the pifton B is deprefled, 

 the lateral hole in the Aiding wire is ftopped by pafling through the collar of leather, and 

 the air below it not being able to return into the receiver, is expelled through the valve g 

 in that pifton. By this time the pifton A has arrived at the^top of that barrel, and the 

 air will have a free paflage into it from the receiver, through the hollow wire, &c. On 

 the depreflioh of this pifton the air below it will be expelled through its valve g, and thus 

 by the alternate elevations and depreflions of the piftons a pretty good exhauftion will be 

 made : at length, however, it will ceafe to take more air from the receiver, in confequence 

 of the fmall quantity of air in the valve holes and under the piftons that cannot be expelled. 

 When this is the cafe, let the cock be turned one quarter round to the left, in which 

 fituation it will make a paflage through the du£t d b, at the fame time that it cuts off the 

 communication between the barrel B and the receiver, and the little air under the valve, &q. 

 in the barrel A will immediately run into the exhaufted barrel B through the du61: d b and 

 hollow wire a a. The pump A will now make a better vacuum, and take air again from 

 the receiver, which will, when the pifton A defcends, be transferred to the barrel B; and, 

 finally, when the pifton B defcends, the air will be expelled through the valve in its 



piftoD. 



