44 



AIR AND FIRE 



to the blast, a design will be made on the glass. The 

 blast is used to clean castings, and will even drill holes into 

 steel. 



The pneumatic hammer is a hammer kept in rapid motion by com- 

 pressed air. It is with this hammer that large pieces of steel, such as 

 those of bridges and of the framework of large buildings, are riveted 



together. 



^Compressed Q<r The air brake, which 



stops the motion of trains, 

 is worked by compressed 

 air stored in the locomo- 

 tive. 



In making foundations 

 for bridges and buildings 

 men must often work under 

 water. They can do this 

 by going down in large 

 caissons, or diving bells 

 (Fig. 36). Compressed air 

 is forced into the bell at 

 sufficient pressure to keep 

 the water from rushing in, 

 and to give fresh air to the 

 workmen. The used air bubbles out around the edge of the bell. 



A submarine boat is supplied with compartments which can be 

 filled with water when the boat is to go under water. The boat also 

 carries compressed air for forcing the water out when the boat is to rise 

 to the surface. 



Diving Bell. 



FIG. 36. 

 How men can work under water. 



44. Exercises. 



Its 



1. What is the volume of the air in a room 40 x 25 x 9 ft.? 

 weight in ounces? In pounds? 



2. If the surface of a man's body is 2,600 square inches, what is the 

 pressure, in pounds, of the atmosphere upon it? Why does not this 

 weight crush the body? 



