GAS PUMPS 



223 



pressed air. Men can also work under water by using 

 diving suits all made of rubber except the head protector, 

 which is made of metal with transparent eyepieces. 

 Some divers carry a tank of compressed 

 air to breathe and others have air pumped 

 to them through a tube. 



Men cannot work under water very long 

 because it is difficult to adjust themselves 

 to the high pressure which is necessary to 

 keep the water out. Divers scarcely ever 

 work at a depth greater than 60 feet, and 

 80 feet or 90 feet is usually considered the 

 limit of safety. But while building the 

 bridge across the Mississippi at St. Louis, 

 Missouri, the diving-bells with the work- 

 men were sunk to a depth of no feet. A 

 case is on record of a diver who sank to 

 a depth of 201 feet while he was investi- 

 gating a wreck off the coast of South America. 



The diver experiences pain in his ears and above the 

 eyes while he is descending and ascending, but he feels 

 no pain when at rest. This is because it takes some 

 time for the air to enter the interior parts of the body 

 and establish a pressure on the inside equal to that on 

 the outside. 



152. The Exhaust Pump. In the exhaust pump the 

 valves are the reverse of those in the compression pump. 

 Air can be pumped from a vessel because it will expand 

 and fill the entire vessel regardless of how much air is in 

 it. When the piston of the pump in the illustration 

 moves upward, the air in the vessel Af, to which the 

 pump is attached, expands and fills the pump tube below 

 the piston; at the same time the air above the piston is 



DIVING SUIT 



