262 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



the caisson is supported and balanced by levers and adjustable 

 tail weights which together form a parallel motion. The rising 

 and falling of the platform is effected by rollers fixed to the 



end of the platform 

 working against curved 

 plates placed in the 

 abutment. 



The displacement of 

 this caisson is 530 tons, 

 and its entire weight 

 294 tons. The weight 

 of concrete ballast re- 

 quired to adjust the 

 caisson was 160 tons. 



Cerro Graving Dock 

 Caisson, Monte Video. 1 

 This caisson (Figs. 

 268 to 271) is of a box 

 form, with keels at- 

 tached to the bottom, 

 which run on rollers 

 placed on the floor of 

 the caisson berth and 

 the chamber in the side 

 walls of the dock en- 

 trance. 



The air-chamber is 

 of such capacity, and 

 the caisson is so bal- 

 lasted, as to bear on 

 the keels with a pres- 

 sure of about 5 tons 

 when the tide is at the 



F IG . 265. level of the water-tight 



deck ; and to krrp this 



pressure uniform, adjusting valves are provided to allow the 

 water to flow into the upper compartment as the tide rises above 

 this deck; these valves are 8 inches in diameter, two on each 

 side of the caisson. 



The keel-rollers set in the floor of the chamber are of 

 1 M.P.I.C.E., vol. Ixv. p. 343. 



