HOIV A MAN WORKS UNDER .WATER 



203 



p 



FIG. 144. The bicycle foot 

 pump. 



raised, the lower valve closes, the upper valve is opened by 



atmospheric pressure, and air from out- 

 side 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 slip around 



it and into the cylinder, but prevents 



its escape from the cylinder (Fig. 144). 

 191. 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 (Fig. 145). 



Blow into the tube and see whether 



you can force the water wholly or 

 | partially down the tube. If the tube 



is connected to a small compression 

 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 compression pump forces 

 air through a tube into the chamber in 

 which men are to work (Fig. 146). 

 The air thus furnished from above sup- 

 plies the workmen with oxygen, and by 

 its pressure prevents water from enter - 

 ing.the chamber. When the task has 

 been completed, the chamber is raised 



and later lowered to a new position. 



FIG. 145. Water does not 

 enter the tube as long as we 

 blow into it. 



