GENOA DOCKS. 



6 inches high ; and above it is the equili- 

 brium chamber, 10 feet 6 inches high; 

 above this chamber there are six regu- 

 lating wells or tanks 10 feet wide and 

 40 to 52 feet long by 38 feet high, open 

 at the top and provided with pipes and 

 valves for admitting and discharging 

 water. These tanks also formed the sup- 

 ports for the staging to carry the work- 

 men, engines, air compressors, cranes, 

 and other appliances. The whole was 

 weighted with 3040 tons of iron ballast, 

 and was floated into position, and sunk 

 or raised by admitting water into the 

 chambers or forcing it out by compressed 

 air. 



The caisson was used only for work- 

 ing in, and no part was left permanently 

 in the work. 



The system of working with this 

 caisson is as follows : When the wells 

 and the equilibrium chamber are empty, 

 the caisson floats and is brought into 

 position ; fey introducing water into the 

 equilibrium chamber, it is sunk until it 

 touches the bottom (in this case, at a 

 depth of 33 feet) ; there will then be 

 about 2 feet 6 inches of water in the 

 chamber. When it is desired to com- 

 mence work, compressed air is forced 

 into the working chamber, and more 

 water let into the equilibrium chamber 

 till the caisson rests firmly on the bottom. 

 The excavation is then began, and the 

 caisson follows it down by the addition 

 of water in the wells. In this way the 

 maximum depth of 50 feet below water 

 level is reached, when the water will 

 stand in the wells at a height of 21 feet 

 above the top of the working chamber; 

 concrete is then filled in through tubes 



Conct*ete 



