. The American railway; its construction, development, management, and appliances . amber in the open bottom is of sufficient height to enablethe laborers to work comfortably in it. This caisson is generallyconstructed upon the shore in the vicinity of the structure andtowed to the point where the foundation is to be sunk. Air is sup-plied by powerful pumps and is forced into the working cham-ber. The pressure of the air of course increases constantly asthe caisson descends ; it must always be sufficient to overbalancethe weight of the water and thus prevent the water from enter-ing the chambe
. The American railway; its construction, development, management, and appliances . amber in the open bottom is of sufficient height to enablethe laborers to work comfortably in it. This caisson is generallyconstructed upon the shore in the vicinity of the structure andtowed to the point where the foundation is to be sunk. Air is sup-plied by powerful pumps and is forced into the working cham-ber. The pressure of the air of course increases constantly asthe caisson descends ; it must always be sufficient to overbalancethe weight of the water and thus prevent the water from enter-ing the chamber. Descent to the caisson is made through a tube, generally ofwrought iron, and having, at a suitable point, an air-lock, which issubstantially an enlargement of the tube, forming a chamber, andof sufficient size to accommodate a number of men. This air-lock yo FEATS OF RAJLJVAY ENGINEERING. is provided with doors or valves at the top and at the bottom, bothopening downward, and also with small tubes connecting the air-lock with the chamber below and with the external air -U—^;- Cancntt Pneumatic Caisson. Entrance to the caisson is effected through this air-lock. Thelower door, or valve, being at the bottom, closes and is kept closedby the pressure of the air in the caisson below. After the air-lockis entered the upper door or valve is shut, and held shut a fewmoments, and the tube connecting with the outer air is closed; asmall valve in the tube connecting with the caisson is then openedgradually and the pressure in the air-lock becomes the same asthat in the chamber below ; as soon as this is effected the valve,or door, at the bottom of the air-lock falls open and the air-lockbecomes really a part of the caisson. A sufficient force of men is employed in the chamber to gradu-ally excavate the material from its whole surface and from underthe cutting edge, and the masonry structure is founded upon thetop of the caisson and built gradually, so as to give constantly asu
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