222 GENERAL SCIENCE 



upward, the valve in the tire closes and prevents the 

 escape of the air from it. The valve V bends downward 

 and allows the air above it to flow below the piston. 

 This air is driven into the tire at the next downward 

 stroke. 



Compression pumps have a very wide commercial 

 use. Every locomotive and street car has a compression 

 pump which supplies the compressed air for the air- 

 brakes; the self-opening and self-closing doors on electric 

 cars are also moved in this way. Compression pumps 

 are sometimes used for ventilating mines so that gases 

 and impure air cannot collect in them; but two rotary 

 fans are more often used for mine ven- 

 tilation, similar to those used for ven- 

 tilating large buildings and factories. 

 One fan forces air into the mine and 

 the other removes it. Compressed air 

 drills used in stone quarries and com- 

 pressed air riveters used in the con- 

 Am KEEPS struction of modern skyscrapers are 

 WATER OUT very valuable commercial tools made 



possible by the compression pump. 

 The compression pump has made it possible for man 

 to work under water. To illustrate how this is done, 

 place a glass tube in water and blow into one end of it. 

 As the air enters, the water moves out, showing that air 

 can hold water out of a vessel when the vessel is immersed 

 in water. A diving-bell large enough for men to work in 

 can be sunk from a boat and air pumped into it to keep 

 out the water; this air also supplies oxygen to the men. 

 Excavations for bridge piers in deep water are sometimes 

 made by sinking an air-tight caisson and then keeping 

 the water out of the place where the men work by com- 



