VOLUME AND PRESSURE 123 



As was seen in Experiment 9, the volume of air can be 

 very much decreased by pressure. It cannot be told from 

 this experiment whether the volume of the gas decreases 

 as the pressure increases or whether it decreases much more 

 rapidly when first pressed upon than afterward. This 

 can be best shown by the use of the Mariotte's tube as 

 in Experiment 52. But if the bicycle pump is a good one, 

 it will answer the question of the rate of decrease quite ac- 

 curately. It is found that the volume decreases directly as 

 the pressure increases. 



Increase of Pressure Due to Decrease of Volume. When 

 a given volume of air is compressed it exerts more pressure. 

 If air is compressed to one third its original space, it will exert 

 three times as much pressure as it did before. When the 

 pressure is removed it regains its original volume. A 

 puncture in an inflated automobile tire shows how rapidly 

 and forcibly air will expand from its greater density under 

 pressure to the density of the surrounding atmosphere. 

 These properties of compressibility and expansion which air 

 has, in common with other gases, have many practical ap- 

 plications. One of the most familiar applications is in the 

 air pumps of garages. Compressed air is also used to apply 

 brakes on street cars, steam engines, and railway coaches. 

 It is used to blow whistles, to ventilate mines and large 

 buildings, and to* operate heavy hammers, rock drills, and 

 riveting machines. 



The force pump illustrates a use of compressed air. An 

 " air cushion " is used to deliver a steady stream of water 

 to a point higher than the mouth of the pump. In the force 

 pump, the water rises into the cylinder when the piston is 

 raised, exactly as in the ordinary lifting pump. The piston 



