Netv CaiiJti-uBtim ifthi Ah-Pamp. ^j-j 



die fame manner as W' a fide valve had been made in Cuthbenfon's barrel, opening 9tit- 

 wards beneath the pifton, and the pifton ufelf had been made folid. For in fucli an arrange- 

 ment the rifing llroke would have drawn the air out of the receiver, and the defcending ftroke - 

 would have driven it out Tideways. But Mr. Sadler has removed the objedions which might 

 be made to fuch a difpofition, by making his pifton in effect fluid, and therefore capable of 

 adapting itfelf to every cavity or irregularity in the vefiel D. 



^ How far it might be found, in procefs of time, that the oil might become changed by the 

 circulation, and Ids fit for the purpofe ; whether it might carry minute bubbl.jb of air along 

 with il^ and whether mercury or any other fluid in vefl'els of fuitable materials might be pre- 

 ferable, I know not. But thus much is certain, that the means of conflruaiiig an inflrument 

 of this kind, with a few flight variations to facilitate the execution, are within the power of 

 many ingenious phllofophlcal men who have neither the ability nor the fltill to procure a 

 good air-pump of the other conllruiSlions. 



Fig. 3 reprefents a barrel in which the Imperfe<Stion of a fpacebetween the pifton and 

 lower valve is obviated, and the adion of this laft valve fecured. A b reprefents the barrel ; 

 O, the pifton rod ; C, the valve in the pifton ; I -I, the leathers ; L L, a Jiollow cylindrical 

 piece of mptal, which nearly fits the cavity M M, which furrounds the lower valve D and 

 its tube. The piece L L does not touch the barrel, and there are two holes, K K, commu- 

 nicating with the central cavity in the pifton, from the fpace between L and the barrel. 

 This pifton alfo is not leathered very tight, becaufe it is intended that the preflure of the 

 atmofphere (hall force a portion of oil into the lower cavity of the barrel during the afcent 

 of the pifton. P is the tube which communicates with the receiver. N is a ftopplng- 

 fcre<V ferving to clofe a hole through which any fuperfluous oil may be drawn. G E is a 

 lever applied to open the lower valve D, when the pifton has rifen to a certain height, and 

 by a proper communication a£ls upon the extremity G by the wire H. Let it be fuppofed 

 that the pifton is down, and that oil is poured into the barrel above it. While the pifton Is 

 drawn up, a portion of the oil will infinuate itfelf paft the leathering Into the partial vacuum 

 below. As foon as the pifton has rifen nearly to Its limit, the valve D will be opened, and 

 part of the air in the receiver will pafs into the barrel through P. Upon the return of the 

 prfton the valve D will in the firft place clofe ; and when the pifton (hall arrive at its lower 

 /htion, the oil in M M, being difplaced by the piece L L, will rife through all the cavltlei- 

 of the pifton, and drive out whatever portion of air may not have efcaped through C during 

 the defcent. And Gnce every rifing ftroke of the pifton will be attended with the tranCtion 

 of a portion of the oil from the upper to the under part of the barrel, the fpace M M 

 ■will, at the end of every defcending ftroke, be found to contain not only a fufficlent quan- 

 tity to (JU the place which would elfe have been occupied by air, but alfo another portion, 

 •«rhich will pafs throogh C at the termination of the ftroke, and complete the effe<2. 



3 L 2 III. Obfervaims 



