Diving Bells and Suits 



369 



exterior allows the excess of compressed air to escape. He can 

 then easily open the outer door and leave the apparatus. 



Fig. 6— (borrowed from the thesis of M. Foley). Diagram representing 

 the construction of a bridge pier by caissons with compressed air. 



As the work progresses and the excavation becomes deeper, 

 the cast-iron tube sinks of its own weight and that of the masonry 

 nn in which it is loaded; then more cast-iron rings are superposed 



