42 



FLUID PRESSURE 



now, this same body of gas expands so that its molecules 

 are farther apart, it is said to be rarefied. 



Experiment 38 If an air pump can be had, any experiments 

 like the following will serve to explain the point. Into the soft 

 bladder of a football allow a small amount of air to enter not 

 enough to fill the ball, by any means. Close the opening tightly 

 and, putting it under the receiver of an air pump, remove the air 

 from around it. Watch the football closely while this is being 

 done. What change occurs in the air within the ball ? How is 

 this change made possible? When you can remove no more air, 

 note the appearance of the ball and admit the air again to the 

 receiver. Explain what now occurs. 



51. The Air Pump. A device for rarefying gases is 

 called an air pump. Fig. 26 shows a common sort. Its 



action is similar 

 to that of the 

 lifting pump, 

 except that the 

 air is sent from 

 the receiver r to 

 the pump barrel 

 c by its own ex- 

 pansive force. 

 As each stroke 

 of the piston p removes some air, the expansive force of 

 that which remains grows less and less until it is no 

 longer strong enough to open the valves a and c. No 

 more air can then be removed, and the vacuum in r 

 will not be perfect. 



A newer form, the mercury air pump, has no valves 

 to be moved by the gas, so that the vacuum formed 



FIG. 26 



