=Jacks.=--The motive power usually employed in driving modern tunnel shields is hydraulic jacks. In some of the earlier shields screw-jacks were used, but these soon gave way to the more powerful hydraulic device. The manner of attaching the hydraulic jacks to the shield is always to fasten the cylinder castings at regular intervals around the inside of the shell, with the piston rods extending backward to a bearing against the forward edge of the lining. In the older forms of shield, having an interior cast-iron reinforcing ring construction, the jack cylinder castings were always attached to this cast-iron ring; but in many of the later shields constructed without this cast-iron reinforcing ring, the cylinder castings are attached to the shell by means of bracket and gusset connections. The number and size of the jacks employed, and the distance apart at which they are spaced, depend upon the size of the shield and the character of the material in which it is designed to work. In stiff and comparatively stable clays, the skin friction of the shield is comparatively small, and an aggregate jack-power of from 4 to 5 tons per square yard of the exterior friction surface of the shield has usually been found ample. The cylinders are spaced about 5³⁄₄ ft. apart, and have a working diameter of from 5 to 6 ins., with a water pressure of about 1000 lbs. per sq. in. In soft, sticky material, giving a high skin friction, the aggregate jack-power required per square yard of exterior shell surface rises to from 18 to 24 tons; the jacks are spaced about 3 ft. apart; and the working cylinder diameter and water pressure are, respectively, about 6 or 7 ins., and from 4000 lbs. to 6000 lbs. per sq. in. With these high pressures, power pumps are necessary to give the required water pressure; but where the pressure required does not exceed 1000 lbs. per sq. in., hand pumps may be, and usually are, employed. Fig. 136 shows the hydraulic jacks used in the East River Gas Tunnel at New York. The number of jacks required depends upon the diameter of the shield, and, of course, upon the distance apart which they are placed. In the City and South London tunnel shield six jacks were used, and in the Blackwall shield 24 were used. The mechanical construction of the jacks for tunnel shields presents no features out of the usual lines of such devices used elsewhere. The jacks used on the East River tunnel shield are shown by Fig. 136.