CHAP, iv.] APPLICATIONS OF COMPRESSED AIH. 



83 



condensing pumps, (these machines are seen in boats to the right 

 of the drawing), air is forced into the two side chimneys, the 

 increasing pressure of the air, greater than the exterior pressure 

 of the atmosphere, will by degrees force out the water which fills 

 the box, and cause it to escape by the open bottom, and will leave 

 the bed of gravel on which it rests quite exposed, not to say dry. 

 The middle chimney alone, which penetrates into the gravel, will 



FIG. 53. Foundation of the piers of the bridge of Kehl by the use of compressed air. 



continue to be full of water. The workmen selected to make the 

 foundations then descend, by the help of chambers forming locks in the 

 lateral chimneys, into the interior of the box thus filled with compressed 

 air. Protected by a pressure of 2 and 3 atmospheres, which guards 

 them against the invasion of the water of the river, they dig out the 

 soil and throw the rubbish towards the base of the central chimney. 

 A dredge, inclosed in this chimney, ascends with its buckets and 



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