When the material is very soft it is often unnecessary to do any actual excavation by hand in front of the diaphragm. The shield is merely pushed forward through the mud or silt and the doors are opened to let the material flow in through them. Workmen dig out this mud and it is hauled out of the tunnel. Whenever bowlders are encountered it is necessary for the men to work outside of the diaphragm to chip away the rocks with compressed-air drills, or else bore them and blast them with small charges of dynamite. In quicksand the material is so fluid that it is unsafe to open the ordinary doors of the caisson, and they are then provided with shutters which are raised one at a time, to permit of operating on a very small section of the head of the tunnel. After enough material has been excavated from in front of the shield, the latter is pushed forward and the excavating is renewed. Unlike caisson work, the weight of the shield is of no assistance in making it penetrate the soil, nor is it possible to move the entire lining of the tunnel with the shield. The tunnel is lined with rings of cast iron which are bolted together, the rings themselves being made up of heavily ribbed curved plates. The shield is formed with a “tail” which fits over the end of the tunnel line like a cap. When the shield is to be pushed forward, a set of hydraulic jacks are fitted between the end of the tunnel lining and the diaphragm of the shield, and by means of these the shield is given a shove forward far enough for a new section of the cast-iron lining to be added within the tail. The tunnel-lining rings are usually sixteen inches wide so that it is customary to move the shield ahead sixteen inches at a time or just far enough for a new ring of lining to be installed. Of course, the tunnel is fitted with air locks by which men can enter the working section without permitting the compressed air therein to escape, and these locks are just like those used in sinking a caisson, except that they are horizontal instead of vertical. QUELLING OCEAN BILLOWS WITH AIR Air is also used in another and very novel way to battle against water. In this case it is not quiet water pressure, but the tremendous power of ocean storms that is combatted. The influence of a shoal upon ocean waves has often been observed. A sand barrier even when submerged to a depth of twenty or thirty feet will break the waves of a heavy storm and leave an area of comparatively quiet water behind it. The reason for this is that the water in the waves does not travel with the waves, but undergoes local oscillatory motion. This motion is in the form of circular or elliptical currents which travel in a vertical plane. When a sand bar is encountered it interferes with these local currents and breaks up the waves.