Two general methods are employed for transmitting the thrust of the piston rods against the tunnel lining. The object sought in each is to distribute the thrust in such a manner that there is no danger of bending the thin front flange of the forward lining ring. In English practice the plan usually adopted is to attach a shoe or bearing casting to the end of the piston rod, which will distribute the pressure over a considerable area. An example of this construction is the shield for the City and South London tunnel. In the East River and St. Clair River tunnels built in America, the tail of the piston rod is so constructed that the thrust is carried directly to the shell of the lining. LINING. Either iron or masonry may be used for lining shield-driven tunnels but present practice is almost universally in favor of iron lining. As usually built, iron lining consists of a series of successive cast-iron rings, the abutting edges of which are provided with flanges. These flanges are connected by means of butts, the joints being packed with thin strips of wood, oakum, cement, or some other material to make them water-tight. Each lining ring is made up of four or more segments, which are provided with flanges for bolted connections similar to those fastening the successive rings. Generally the crown segment is made considerably shorter than those forming the sides and bottom of the ring. The erection of the iron segments forming the successive rings of the lining may be done by hand in tunnels of small diameter where the weights to be handled are comparatively light, but in tunnels of large size special cranes attached to the shield or carried by the finished lining are employed. The construction of the iron lining for the Hudson River tunnel is illustrated in Chapter XX., and that for the St. Clair River tunnel is shown by Fig. 137. [Illustration: ~Part Transverse Section.~ ~Longitudinal Section.~ FIG. 137.--Cast-Iron Lining, St. Clair River Tunnel.] CHAPTER XX. SUBMARINE TUNNELING (Continued). THE SHIELD AND COMPRESSED AIR METHOD. THE HUDSON RIVER TUNNEL OF THE PENNSYLVANIA RAILROAD. The shield and compressed air method of excavating subaqueous tunnels is used when the distance is small between the roof of the tunnel and the bed of the river. These tunnels are usually driven from the shafts sunk from each shore. It is very seldom they can be driven also by an intermediate shaft. This, however, was done in the case of the Belmont tunnel under the East River. Here the tunnels passed under the man-of-war reef where a working shaft was sunk.