33* 



PUMPS, AND THEIR VALUE TO MAN 



enters the cylinder. The next stroke of the piston drives a 

 fresh supply of air into the tire, which thus in time be- 

 ( comes inflated. In most cheap bicycle 



pumps, the piston valve is replaced by 

 a soft piece of leather so attached to the 

 piston that it allows air to flow into 

 the cylinder, but prevents its escape 

 from the cylinder (Fig. 201). 



How a man works under water. 

 Place one end of a piece of glass tube 

 in a vessel of water and notice that the 

 water rises in the tube. Blow into the 

 tube and see whether you can force the 

 water down the tube (Fig. 202). If the 



FIG. 202 . -Water does not tube is connected to a small compres- 

 enter the tube as long as sion pump, sufficient air can be sent 

 into the tube to cause the water to sink 



and to keep the tube permanently clear of water. This is, in 

 brief, the principle employed for work under water. A com- 

 pression pump forces air 

 through a .tube into the 

 chamber in which men are to 

 work (Fig. 203). The air 

 thus furnished from above 

 supplies the workmen with 

 oxygen, and by its pressure 

 prevents water from entering 

 the chamber. When the 

 task has been completed, the 

 chamber is raised and later FIG. 203. The principle of work under 



. , . . water. 



lowered to a new position. 



Combination of pumps. In many cases the combined use 

 of both exhaust and compression pumps is necessary to secure 



