224 



GENERAL SCIENCE 



EXHAUST PUMP 



being forced out. When the piston moves downward 

 the valve D closes and does not permit any air to enter, 

 while the air below the piston passes around the piston 

 valve V and fills the pump tube above the piston. At 

 each upward stroke a pump full of 

 rarefied air is taken from the vessel. 

 The commercial use of the ex- 

 haust pump is not so extensive as 

 that of the compression pump, al- 

 though very necessary. Electric 

 light bulbs have the air removed 

 so that oxygen cannot burn the 

 filament. Thermos bottles have the 

 air removed from between the two 

 glass bottles so that heat cannot 

 pass out by conduction or convection. 

 153. Pneumatic Dispatch Tubes used in department 

 stores, railroad stations, and in many cities for sending 

 mail, have an exhaust pump at one end to remove air 

 from the tube. The articles sent through the tubes are 

 placed in leather cases which fit tight in the metal tube 

 so that the air cannot pass around them. The exhaust 

 pump removes the air pressure from in front of the 

 leather case, and the compressed air behind it forces 

 the case rapidly through the tube. 



QUESTIONS AND EXERCISES 



1. Take the valve out of a bicycle pump and explain its action. 



2. Is the bicycle pump a compression pump or a force pump ? 



3. How does a compression pump differ from an exhaust pump ? 



4. What practical use is made of both kinds of pumps ? 

 0. Which kind do you think is used more extensively ? 



