Chap, xxiii.] THE AIR PUMP. 281 



piston, When the piston descends the pressure closes 

 the receiver valve, and prevents the air being driven 

 back, and it, at the same time, opens the piston valve 

 and permits the. escape of the air outwards ; when the 

 piston again ascends its valve closes, and a further 

 quantity of air is withdrawn from the receiver. With 

 each movement of the pump only a fraction of the air is 

 removed, the gas becoming more and more rarefied, 

 because, owing to its elastic property, it expands to 

 occupy the space. W r ith each stroke the quantity 

 removed, therefore, diminishes, and a perfect vacuum 

 can never be produced in this way, because it is 

 always just a fraction of the rarefied air that is with- 

 drawn. There is a limit, then, which cannot be passed. 

 It will be readily understood, that, as the rarefaction 

 proceeds, the two sides of the piston will be under 

 different pressures ; the outer side under atmospheric 

 pressure, and the inner side under the pressure of the 

 rarefied air, the former greatly preponderating. Every 

 upward movement of the piston will be made with in- 

 creasing difficulty against the atmospheric pressure. 

 This is overcome by using a two-barrelled pump, (as 

 in the figure) the pistons being worked by a horizontal 

 lever, so that one is up when the other is down. The 

 hindrance by pressure to the upward movement of 

 one is balanced by the aid to the downward movement 

 of the other. To indicate the degree of rarefaction one 

 limb of a bent tube containing mercury, opens into 

 the tube connecting barrel and receiver, the other 

 limb bein closed. The difference in the level of 



O 



the mercury in the two limbs indicates the pressure ; 

 the more nearly the two columns of mercury are of 

 the same level, the more nearly perfect is the vacuum, 

 for the elastic force of the gas acting from the receiver 

 would force the mercury down in the open limb 

 and up to the top of the closed limb. Consequently, as 

 this elastic force is reduced by the rarefaction of 



