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pounds and is carried by ~>\- steel columns. This 

 enormous weight is carried down to rock, at an aver- 

 age depth of about 90 feet below curb, by 30 caissons. 

 These are Dimply airtight bottomless boxes, square 

 or cylindrical in cross section, having interior cham- 

 bers large enough for a gang of men to enter and 

 excavate the bottom. The excavated material passes 

 up through steel shafts in the roof. As the caisson 

 extends below the waterline, the compressed air is 

 pumped in to expel the water from the open lower 

 or cutting edge. As the caisson descends there is a 

 greater pressure of air required, which is supplied 

 continuously to the men through an ordinary pipe 

 leading into the working chamber. 



This pressure is just sufficient to balance the 

 weight of the water on the outside and thus prevent 

 the water from rushing into the working chamber 

 and taking material from under the adjoining build- 

 ings and streets along with it. 



As water weighs about (te pounds per cubic foot, 



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the pressure at one foot depth is .434 pounds per 

 square inch, or a little less than one-half pound for 

 each foot of depth, or 44 pounds per square inch 

 if the water were 100 feet deep, which is about the 

 limit of human endurance and is in addition to the 

 atmospheric pressure of about 1.5 pounds per square 

 inch. In the higher pressures the risk of one's los- 

 ing his life or being paralyzed for life is very great. 



The caisson is thus sunk by undermining, by shov- 

 eling the material into one-half cubic yard buckets, 

 aided by a heavy weight of concrete, which is added 

 over the roof of the caisson as the latter gradually 

 sinks, and also by additional pig iron blocks, of which 

 as much as 1,200 tons were used on this job. When 

 it has reached a satisfactory hard stratum, which is 

 cleaned and leveled, the whole interior of the caisson 

 and of the shaft connecting the working chamber 

 with the outer air, is filled with rammed concrete, 

 forming a solid monolith upon which the superstruc- 

 ture is readily supported. To permit the passing in 



