SOME CHARACTERISTICS OF THE AIR 



3 



the doors leading to the caisson and enters, closing the doors 

 behind him. When he emerges from the caisson the process is 

 reversed. In practice 

 it is found necessary to 

 allow a man to remain 

 in the intermediate 

 chamber, or air lock, for 

 some time, while the air 

 pressure is slowly in- 

 creased or decreased in 

 order that he may not 

 suffer from too sudden 

 change of pressure. 



Compressed air is 

 used also for excluding 

 water in excavating 

 tunnels beneath water 

 level. The principles 

 involved in excavating 

 tunnels are similar to 

 those of the caisson. 

 Diving bells are an- 

 other form of apparatus 

 in which air is used to 

 exclude water from the 

 working space when 

 men work under water. 



3. The weight of air. 

 Not only is air like 

 other substances in oc- 

 cupying space, but, like 

 them, it has weight. 

 The best way to show 

 that air has weight is to weigh a quantity of it, as may be 

 done in the following manner: The air (almost all of it) 



FIG. 3. Working under water by means of 

 an air chamber, or caisson 



A sectional view of the caisson used in sinking 

 the foundation for the lighthouse shown in 

 figure 2. Note the men working in the air cham- 

 ber below; also the three men in different 

 stages of passing into the air chamber through 

 the air lock. After a drawing by the United 

 States Bureau of Lighthouses 



