The men at the front attack the rock with air drilling machines and charges of dynamite. The holes are driven at a smaller depth than in land work; very light charges of dynamite are used and only a few holes fired at each round. Every precaution is taken in order not to disturb the shield and the bed of the river any more than is possible, because at a shallow depth the blast would tend to widen the existing crevices in the rock and thus permit an inflow of water. When the rock is fissured or disintegrated and the roof of the excavation at the front requires timbering, the shield should be kept closer to the front. In this way the quantity of timber for strutting is greatly reduced, so lessening the probabilities of fires. It is very difficult, in compressed air, to extinguish fires and in almost every instance the only way is to flood the tunnel. This was done at the Manhattan end of the tunnel under the East River for the extension to Brooklyn of the New York Subway. The excavation is made by hand in loose but compact soils such as clay. The men work on platforms located at the front of the shield and they are protected from the caving-in of the roof by a hood added for working through loose soils. The men excavate the material which is shoveled inside the tunnel and is carried away in small cars. The shield is very close to the front of the excavation in loose soil. The East Boston tunnel, under Boston Harbor, connecting with the Boston Subway, was excavated through blue clay. The minimum distance between the bottom of the water and the roof of the excavation was 18 ft. The tunnel was excavated by means of compressed air and the shield which was only used for the roof. It slid on top of concrete side walls built in two drifts which were excavated nearly 100 ft. ahead of the shield. The tunnel was lined with concrete, the arch being reinforced by longitudinal steel rods which received the thrust of jacks used for advancing the shield. The material in the drifts under the shield and the bench was removed by hand and carried away in small cars. Subaqueous tunnels driven through very loose soils can be excavated by simply leaving the doors open while the shield is pushed ahead. The material, dislodged by the cutting edge of the shield, is forced through the doors and falls on the floor whence it is removed in small cars. In very loose soils the excavation has been made in a still more economic way; the shield with closed doors is simply squeezed through the soil. This method is financially convenient, because all the excavating and hauling operations are eliminated and the tunnel progresses from 40 to 50 ft. per day, but clearly indicates a lack of stability. In this manner, the Hudson River tunnel of the New York and New Jersey Railroad was constructed.