CHAPTER XXIII 

 GAS PUMPS 



150. Gas Pumps are used principally for pumping air. 

 The structure of the gas pump is different from that of 

 the liquid pump. The valves are different, and to be air- 

 tight they must fit more accurately than to be merely 

 water-tight. In cheap pumps the valves are usually 

 made of leather and are kept soft and flexible by lubri- 

 cating oil. Since the molecules of air are moving very 

 rapidly, it will expand when the pressure on it is reduced. 

 On this account air can be pumped into or out of a vessel. 

 To pump air into a vessel the valve on the piston must 

 be turned in the opposite direction from that in a pump 

 that will force air out of a vessel; the former is called a 

 compression pump and the latter an exhaust pump. 



151. The Compression Pump. The compression 

 pump is one with which air can be compressed or forced 

 into a vessel or bicycle tire. The diagram 

 illustrates the action of a compression pump. 



When the piston is forced down, the valve 

 V rubs against the side of the pump tube 

 and prevents the air from escaping above or 

 around the piston, and so the air is forced 

 through the rubber tube into the tire. While 



the piston was going downward, the pump 



' , , . , . -,, -, .,, COMPRESSION 



tube above the piston was being filled with puMp 



air which passed in through the opening 0. 

 Some pumps permit the air to pass in through the open- 

 ing around the piston bar. When the piston is drawn 



o 



