124 THE ATMOSPHERE AND ITS SERVICE TO MAN 



has no valve, and so when it descends it forces the water 



out through the pipe (E) (Figure 67) into the air chamber 



(D), thus compressing the air 

 in it. The valve (C) keeps 

 the water from running back 

 when the piston is lifted. 

 While the piston is ascend- 

 ing, the pressure of the air 

 cushion (D) forces a steady 

 stream through the pipe (^4) 

 to the tank above. 



The force pump is some- 

 times used to fill tanks in 

 attics of farmhouses so as 

 to provide private water- 

 systems. The principle of 

 the force pump is used in 



the more complicated pumps for water-works, fire engines, 



and mines. 



Heat Produced by Compression and Cooling Produced by 

 Expansion. Experiment 63. Have a five-pint glass bottle fitted 

 with a two-hole rubber stopper. Pass through the 

 holes in the stopper a chemical or air thermometer and 

 a short glass tube. The lower end of the glass tube 

 which extends into the bottle should be kept as far as 

 possible away from the bulb of the thermometer, so 

 that when the air is exhausted or allowed to enter the 

 bottle there will be no movement of the air near the 

 bulb of the thermometer. The end of the column of 

 the thermometer must be visible above the stopper. p IQUBE 

 (Figure 68.) 



Attach the glass tube to an air pump by means of a thick-walled 

 rubber tube. Note the temperature of the thermometer within 

 the bottle and also of the air outside. Quickly exhaust the air 



FIGURE 67. DIAGRAM OF A 

 FORCE PUMP 



