340 LECTURE XXIX. 



length of time, the limit of its perfection will be a rarefaclion expressed 

 by the proportion of the air remaining in the barrel, when the piston is down, 

 to the whole air that the barrel is capable of containing ; for such will be the 

 rarity of the air in the barrel when the piston is raised. It becomes, there- 

 fore, of consequence to lessen the quantity of this residual air as much as 

 possible; and at the same time to take care that the valve may be capable of 

 being accurately closed and easily opened, or that a stojicock may be occa- 

 sionally substituted for it, which may be opened and shut by external force, 

 when the elasticity of the air remaining is too small to lift the valve. In 

 pumping water from a well, we raise an equal quantity at each stroke, but in 

 the air pump, we withdraw at most only equal bulks of the air diiferently rare- 

 fied, so that the quantity extracted is continually diminished as the operation 

 proceeds. Thus, if one tenth of the air were exhausted by the first stroke, only 

 nine tenths as much, that is, one tenth of the remainder, would be drawn out 

 by the second ; hence, in order that the process may be expeditious, it is 

 of importance to have the barrel as large as possible in proportion to the 

 receiver. In cases where the presence of aqueous vapour would be of no 

 consequence, the exhaustion might be made very rapidly by filling the whole 

 apparatus with water, which was the method first employed by Otto von 

 Guericke, the inventor of the modern air pump. 



In order to lessen the labour of the operation, two barrels may be em- 

 ployed, and so connected as to work alternately ; in this manner the pressure 

 of the atmosphere, acting on both pistons at once, opposes no resistance to 

 their motion in either direction. In Smeaton's pump a single barrel has 

 nearly the same advantage, the rod of the piston working in a collar of 

 leathers with oil, and the air being excluded from the upper part of the 

 barrel by a valve, through which the air passes when the piston is raised 

 near to the top ; so that in the descent of the piston there is a vacuum 

 above it, and the air below opens the valve much earlier, and passes more 

 completely through it, than in the common air pump; and the piston is only 

 exposed to the whole pressure of the atmosphere during the discharge of the 

 air through the upper valve. (Plate XXIV. Fig. 3'^25.) 



That the air is really removed by the operation of the air pump, may be 

 demonstrated by various experiments, which show the absenceof its resist- 



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