The plant is located at some convenient point near the head shaft. It consists of a set of boilers to provide the power for the different machines. They are low and high pressure compressors, the former supply the air through the tunnel; the latter, the air for working the drills, in case rock is encountered, and power for hauling and hoisting purposes. The various pumps force the water for the hydraulic rams that drive the shield and work the erector. They also remove the water from the tunnel which always collects in variable quantities at the bottom of the excavation. Besides the machines for light and ventilation purposes, the head shaft is provided with an overhead construction where are housed the hoisting machines, the telephone and other means of communication with the work at the front. Usually a long trestle is built in connection with the head shaft, leading to the dumping place and yard. On this inclined elevated structure are located, also, the tracks upon which will run the small cars used inside the tunnel for hauling purposes. The shafts are excavated on a square, rectangular or circular plan and are usually lined with masonry. It is only recently that shafts excavated through loose soils have been lined with the same cast-iron lining used in the tunnels, the only difference being that the rings were laid flat on the ground and attached to those already sunk. After the shaft has been sunk to the required level, the tunnel is driven toward the river by any one of the methods used for land work. At some convenient distance from the shaft, the dimensions of the tunnel are enlarged for a length of 20 or 30 ft. In this larger space, called the shield chamber, the shield is assembled, mounted, and, when completed, it is slowly pushed toward the river. The tunnel is excavated from the shield chamber on, with dimensions equal to the exterior shell of the shield. The construction of the shield and the hydraulic jacks used for its advance are explained in a preceding chapter.