COMBINATION OF PUMPS 205 



carries on his back a tank of compressed air, from which the 

 air escapes through a tube to the space between the body 

 and the suit. When the air has become foul, the diver 

 opens a valve in his suit and allows it to pass into the water, 

 at the same time admitting a fresh supply from the tank. 

 The valve opens outward from the body, and hence will allow 

 of the exit of air but not of the entrance of water. When the 

 diver ceases work and desires to rise to the surface, he signals 

 and is drawn up by a rope attached to the suit. 



192. Combination of Pumps. In many cases the combined 

 use of both exhaust and compression pumps is necessary to 

 secure the desired result ; as, for example, in pneumatic dis- 

 patch tubes. These are employed in the transportation of 

 letters and small packages from building to building or be- 

 tween parts of the same building. A pump removes air 

 from the part of the tube ahead of the package, and thus re- 

 duces the resistance, while a compression pump forces air 

 into the tube behind the package and thus drives it forward 

 with great speed. 



