AIR Pt7MP. 307 



receiver and barrel, and Ihe air by its elaftic force rufties into The latter is 

 the ban-eland fills; it. The pifton now returns towards the^^* al P nng 

 duct B, and drives before it the air contained in the barrel, 

 together with the fpring pifton N, until they are flopped by 

 the fhoulder D at the inftant in which the ends of the two pi- 

 lions comeagainft the middle of the duel B. By forcing the 

 air out at the duel B, thepiftons come into contact, and form 

 one uniform cylinder, that prevents any communication of the 

 barrel with the external air. The pifton P is now drawn back 

 toward the duct A, and the fpring pifton N, by the action of 

 its fpring, follows in clofe contact with it, until it is (topped by 

 its ihoulder G meeting with the end of the barrel, after having 

 palfed the duel B, and having continued to intercept the com- 

 munication between the barrel and (he external air. This is 

 the fitualion with which the defcription began ; and, repeating 

 the operation, when the pifton P is drawn back beyond the 

 duel A, the air from the receiver rufhes into tiie barrel; and 

 when it moves forward to the duel B this air is expelled. 



Having exhibited a general defcription of this pump, with 

 the manner of its working, a more particular illuftration of 

 fome of its parts will be given. 



When the pump is intended to exhauft, the receiver muit This pump ex- 

 be placed over the dud A, leaving the du& B open to the ex- J™£j °J co { n ^ 

 ternal air j but when it is defigned to condenfe, nothing more Cure. 

 is neceflary than to thift the (ituation of the receiver on the 

 plate, placing it over the duel B, and leaving the duel A open 

 to the external air. 



The duel B is continued around the fpring pifton by means 

 of a circular channel cut into the infide of the barrel, in order 

 that the air might efcape from all fides when the piftons come 

 into contact. 



It may be obferved that all the back fpace in the barrel be- Obfervations« 

 tween the collar O and the pifton P makes a part of the ca- 

 pacity of the receiver; or, to fpeak more accurately, the 

 fpace O A between the collar O and the duct A : the fpace 

 A P between the duel and pifton, while it moves from A toward 

 B, being only a temporary dilation of the capacity, and the 

 fpace A P while it moves from A towards O a temporary con- 

 traction of it. 



For the purpofe of preventing a fluctuation of the air in the Advantage of 



receiver, which would be caufed by this expanfion and con- ■ co . n ? ruftl . ng !t i 



■ r with two bands. 



X 2 traction, 



