156 PNEUMATICS. 



In all mercurial experiments with the air-pump, 

 a short pipe must be screwed into the hole of the 

 pump-plate, so as to raise above it about half an 

 inch, to prevent the quicksilver from getting into 

 the air-pipe and barrels, in case any should acci- 

 dentally be spilt over the jar ; for if it gets into 

 the barrels, it spoils them, by loosening the solder, 

 and corroding the brass. 



With respect to the leathers, if the pump-plate 

 is not ground, they are absolutely necessary ; they 

 should be previously soaked in oil, from which the 

 moisture has been expelled by boiling, or hog's- 

 lard, with a little bees-wax, which gives a clammi- 

 ness very proper for the purpose. 



It is evident, that the vacuum in the receiver of 

 the air-pump can never be perfect, that is, the air 

 can never be entirely exhausted ; for it is the 

 spring of the air in the receiver that raises the 

 valve, and forces its way into the barrel ; and the 

 barrel at each suction can only take away a certain 

 part of the remaining air, which is in proportion 

 to the quantity before the stroke as the capacity 

 of the barrel is to that of the barrel and receiver 

 added together. 



This, however, is an imperfection that is seldom 

 of much consequence in practice ; because most 

 air-pumps, at a certain period of the exhaustion, 

 cease to act, on account of their imperfect con- 

 struction ; for the valves usually consist of a piece 

 of oiled bladder, tied over a hole j so that the air is 

 at liberty to pass by lifting up the bladder, but 

 cannot return again : and thus there will unavoid- 

 ably be a small space left between the lower valve 

 and the piston, when down. 



Now, it will happen, when the air in the re- 

 ceiver is very rare, that its spring will not be 

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