Access to the working chamber is had through a central shafting. In order to hold the air pressure in the working chamber and yet provide for the entrance of men and materials, an air lock is fitted to the upper end of the shafting. This air lock, as shown in Figure 43, consists of a chamber formed with a trapdoor at the top and the bottom. Both doors open downwardly. To enter the caisson, the bottom door of the chamber must first be closed and means are provided for doing this from outside the air lock. The compressed air in the chamber is then let out through a valve, and when the pressure drops almost to normal the upper trapdoor falls open of its own weight, giving access to the chamber. After entering, the upper door is closed while compressed air is admitted into the chamber; the air pressure then serves to hold the upper trapdoor closed. The bottom door, in the meantime, has been kept closed by the air pressure below, which is greater than that above the door. But after sufficient air has been introduced into the chamber to equalize the pressure on both sides of the lower trapdoor, it falls open of its own weight. The occupants of the chamber can then proceed down a ladder to the working chamber. When leaving the caisson, the action of the air lock is reversed. The bottom door is pushed up and held closed for a moment while air is released from the chamber, when the greater pressure beneath will hold the door closed, and after the pressure within the air lock has been lowered practically to normal, the upper door drops of its own weight, permitting the occupants of the chamber to climb out. The same process must be undergone by buckets loaded with sand or earth from the excavation and by empty buckets returning to the working chamber. More time is required for a man to pass through an air lock than for a load of sand or any inanimate load. In some of the larger caissons a separate small air lock is provided just for the use of the workmen. In the air lock the pressure must be built up slowly so that it will permeate a man’s whole system. When we realize that the pressure that men have to support in caisson work may amount to from fifty to one hundred tons on the whole body, it is difficult at first to understand why the body is not flattened out like a pancake. It is only by permitting the system to absorb the pressure so that there is as much internal pressure as that outside that a man is able to enter a compressed air chamber without harmful results. THE CAISSON DISEASE