It is comparatively simple to sink a vertical shaft into water-bearing soil, but a horizontal shaft involves serious difficulties. The action of a diving bell is easily illustrated by inverting a tumbler and pressing it down into a basin of water. The air trapped in the tumbler will keep the upper part of the glass dry, and by inserting a tube in the tumbler it is possible to fill the tumbler so full of compressed air as to drive out all the water. This is virtually what is done in the caisson; but when excavating a horizontal bore, the caisson must be turned on its side. Turn the tumbler on its side and it is impossible to keep the water out of it, no matter how much air we may blow into it. The reason for this is that the pressure on the open end of the tumbler is not uniform. At the bottom, where the water is deeper, it will be greater than at the top. If air is pumped in to equalize the water pressure at the upper edge of the glass, it will not prevent water from flowing in at the bottom; and if it be equal to that at the bottom, the water pressure at the top cannot hold the air in and keep it from pouring out. [Illustration: FIG. 44.--SECTIONAL VIEW OF A TUNNEL SHIELD] Fortunately most of the soil through which a subaqueous tunnel is driven is not very fluid. It is either sticky, as in clay, or sluggish enough to prevent the water from flowing in rapidly. If there is enough cover of silt or earth above the tunnel bore, it will help to hold the air in the tunnel. When the bore comes very close to the surface of the bed of the stream that is being tunneled, loads of clay are dumped along the line of the tunnel to provide the requisite cover. In tunnel boring a shield is used which is the equivalent of the caisson in vertical boring. The shield is a cylindrical box with a diaphragm across it corresponding to the deck of the caisson. (See Figure 44.) In front of the diaphragm there is a small working chamber which is protected above by an extension of the shield known as an apron. In the diaphragm there are a number of doors at different levels, which may be closed in case of danger. If work is proceeding near the top of the shield, the upper doors are opened and the pressure is regulated to equal the water pressure at that level. If the work is carried on near the bottom of the shield, the upper doors are closed and only the lower doors are open, and the pressure is increased to equal the water pressure at that point. Sometimes the material is of such a nature that the men can safely pass out of the doors into the working chamber outside, but more often it is possible to work only within a limited area immediately in front of the doors.