6 ELEMENTS OF GENERAL SCIENCE 



The water is kept out of the caisson by the use of air pumps 

 which force air into it with sufficient pressure to overcome 

 the pressure of the water. The entrance and exit of work- 

 men without allowing the escape of the air in the caisson is 

 made possible by means of a tubular passageway which leads 

 to the bottom of the caisson. 



When a workman wishes to enter the caisson, he first goes 

 into the tube through an upper door, which is tightly closed 



after his entrance. 

 Air is then forced 

 into the tube "or 

 " air lock" until its 

 pressure is equal to 

 that in the caisson/ 

 He then opens the 

 doors leading to 



__ the caisson, and 



enters, closing the 

 doors behind him. 

 When he emerges 

 from the caisson, 



the process is re- 

 FIG. 5. A lighthouse upon a submerged foundation verge(i> j n pract i ce 



The foundation was constructed by men who worked ., . t 



under water lfc ls f OUnd neceS ' 



sary to allow a 



man to remain in the air lock for some time, while the air 

 pressure is slowly increased or decreased, in order that he 

 may not suffer from too sudden change of pressure. 



Compressed air is used also for excluding water in exca- 

 vating tunnels beneath water level. The principles involved 

 in excavating tunnels are similar to those of the caisson. 

 Diving bells are also used when men work under water. The 

 use of compressed ah- in raising torpedoed ocean vessels gives 

 promise of making it possible to regain immense quantities 

 of lost property. 



