THE ATMOSPHERE 



121 



FIG. 45. Caissons. A , the working chamber where excavation goes on in 

 an atmosphere of such density (and pressure) that water from without is kept 

 from entering; B, air-tight shaft through which workmen and material 

 pass up and down; E, E', chambers through which workmen pass to and fro 

 from the cage, and where the atmospheric pressure to which they are sub- 

 jected is gradually changed. F, F r , ballast of masonry to keep caisson in 

 place; G, pipe conveying^air under great pressure into the caisson chambers. 



