258 OUTLINES OF NATURAL PHILOSOPHY. 



348. The Air-pump is a machine by which the 

 air is extracted from close vessels. A cylindric 

 barrel in which a piston works, communicates by 

 a valve in its bottom, which opens inwards, with 

 the receiver or close vessel. The piston is also 

 furnished with a valve, which opens outward ; so 

 that when the piston is thrust down, the air in the 

 barrel opens the valve of the piston, and makes its 

 escape. After the piston ! has been forced down to 

 the bottom, it is drawn up, when its valve shuts, 

 and that in the bottom opens, and admits the air 

 from the receiver. This operation is repeated, 

 and the air in the receiver becomes continually 

 rarer. 



The air-pump was invented by OTTO GUERICKE, a 

 gentleman of Magdeburg in Germany, about the 

 year 1654, and marks a great era in the his- 

 tory of the physical sciences. It has received 

 great improvements. CAVALLO, vol. n. p. 465., &c. 

 BUTTON'S Mathematical Dictionary, article Air- 

 pump. 



349. When the number of strokes of the piston 

 increases in arithmetical progression, the quantity 

 of air remaining in the receiver, and also its 

 density, form a series of terms decreasing in 

 geometrical progression ; so that if R be the 

 capacity of the receiver, B of the barrel, D 

 the density of the air in the receiver, after n 



strokes 



