Compressed Air Work And Diving 1909

KEEPING TO LINE AND LEVEL. 45 points should be checked by reference to a more perma- nent bench mark once or twice a week, and more often if the staging shows any signs of sinking, as it very frequently does. The reducecl level of the cutting edge is easily obtained, as the height of the various horizontal joints can be taken from the working drawings. If there are no suitable horizontal joints, the four corners of the caisson should be clearly marked every foot in height. 11 is of importance that the caisson should be pitched, not only level, but in the correct position. If it should get out of position and is not too far in the ground, it Fig. 2.—Method of Strutting Caisson. may be brought back by fixing wooden struts against the walls of the working chamber in the clirection in which it is required that the caisson shall move (Fig. 2). As the caisson sinks, the tops of the struts will move in the arc of a circle, and thus the caisson will be forced to move in the required clirection. This method must, however, be usecl with great caution, or else the walls of the working chamber may be injured. If the caisson is in rock, it will be necessary each time it is lowered to clear well behind the cutting edge alono- the side which is to move outwarcls, or else it will