Centuries ago it was realized that it would be possible for men to descend into the open sea if they were protected by a bell-shaped chamber, for the air trapped in the chamber would furnish them with oxygen requisite for breathing purposes, and would prevent the water from drowning them out. This same idea of a diving bell is used on land when sinking a shaft through quicksand or water-bearing strata. A large box or caisson is used. (See Figure 43.) This box may be either cylindrical or rectangular, and it is open at the top and bottom. The lower edges of the box are shod with steel and form cutting edges that will sink into the soil that is being excavated. At a height of about seven feet above the bottom of the box there is a transverse diaphragm known as the deck, and the space below is known as the working chamber. This deck is very strongly constructed, as it has to support the weight of the concrete shaft that is built above it. Laborers, commonly known as “sand hogs,” enter the working chamber and dig out a shallow pit in the floor of clay or sand. This pit is then extended to the cutting edges of the caisson. The caisson thus undermined settles down into the excavation and another pit is started. In this way, step by step, the caisson is sunk into the ground. In order to overcome the friction of the caisson against the sides of the excavation and to insure its sinking, it is heavily weighted. When water-bearing sand is reached, compressed air is admitted into the working chamber to force the water out. When the air pressure is greater than the water pressure it drives the water out of the sand in the working chamber, so that the men can work in perfectly dry ground, even though the surrounding sand may be so saturated with water as to form a quicksand. Of course the farther the excavation proceeds below the water level the greater the air pressure required, and the caisson would be blown up out of the ground by this air pressure or would float on the water were it not for the weights with which it is loaded. When an open shaft is to be dug, pig iron is loaded on the caisson to force it down, but in most work the object is merely to sink a concrete column down to rock and so the caisson is filled with concrete above the deck. Sections are added to the caisson as it sinks into the ground, and these sections are filled with concrete. This method of building the column facilitates the work of laying the concrete, and at the same time provides the weight necessary to overcome the buoyancy of the caisson and the skin friction on the side walls of the excavation. After the caisson has been carried down to rock and a good seat has been blasted out of solid rock, the working chamber is completely filled with concrete and the concrete shaft is thus anchored to the rock. THE AIR LOCK