THE SLING, BOW, AND OTHER WEAPONS 145 



the upstroke, but closes when the down stroke begins. A second 

 valve lets the air out of the cylinder on the down stroke into the 

 tire, As more and more air is pumped into the tire the elasticity 

 of the air increases and so the pressure against the inner wall of 

 the tire is greater and greater. When the valve is open between 

 the cylinder of the pump and the tire, the air in the pump is 

 exerting the same pressure on the plunger and walls of the cylinder 

 as is exerted on the walls of the tire. If the plunger were larger 

 the upward pressure upon it would be difficult to overcome, and 

 it would take more power than the average person has to force 

 the plunger down. Sometimes the automobile pump is made 

 of two cylinders with a plunger in each. The upstroke of the 

 plunger in the large cylinder drives the air into the small cylinder, 

 and the down stroke drives the somewhat compressed air into 

 the tire. The cross-section of the plunger in the small cylinder 

 may have an area of, say, only one-quarter square inch. It 

 would have to be pushed down, therefore, only with a force 

 slightly exceeding 15 pounds to overcome a pressure in the tire 

 of 60 pounds per square inch. Yet the large cylinder has capa- 

 city enough so that the tire can be pumped up quite rapidly. 



