[ 379 1 



no air beiag ever admitted into the barrel through the valve 

 or through the perforation in the pifton : for if when the 

 valve is open, difcharging the air ptimped out, and the ex- 

 ternal air is then prefling on the pifton with its whole weight, 

 any of it fhould force its way through the pifton ; the rec'^ 

 is then open towards the barrel, and it muft pafs in an in- 

 calculable quantity into the rec". Now the pifton in his pump 

 has the du(5t through it opened at every ftroke, and it is 

 clofcd again by a conical ftopple, fupplying the place of a 

 cock, and made air-tight only by oil ; which ftopple being 

 faftened clofe in the pifton only by the force with which 

 the pifton is raifed, arifing from friiflion, this force or pref- 

 fure (unlefs the pifton moves more ftiflly than one lubricated 

 with abundance of oil, may be fuppofed to do) may fome- 

 times be infiifficient to make the ftopple firm in its focket, 

 until it is gradually prefled by air condenfed above it, and 

 until the valve opens, and the external air refts on the pifton ; 

 fo that if the pifton be not perfedly alr-faft, fome of it muft 

 get through into the rec' ; and though the fridlion of the 

 pifton fliould be always fo great as to prevent this happening, 

 yet if any fmall particle of dirt fliould have got between the 

 ftopple and its cavity, it would remain impaded there, and be 

 likely to render the ducH: permeable to air : it feems therefore 

 too delicate a clrcumftance, to reft the performance of the 

 machine on, that this dudl enclofed within the barrel, and 

 opened at every ftroke, flaould, at once, be fo clofely fhuf 



3 B 2 again^ 



