SI 'KM Alt INK Tl NNKUNG 265 



Jacks. -- The motive power usually employed in driving 

 modern tunnel shield* is hydraulic jacks. In some of the 

 earlier shields screw-jacks weie 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 construc- 

 tion, the jack cylinder eastings were always attached to this 

 cast-iron ring; but in many of the later shields constructed 

 without this cast-iron reinforcing r ' n g> ^ ie cylinder castings 

 are attached t<> the shell by means of bracket and gusset con- 

 nections. The nuinlx'r and size of the jacks employed, and tin? 

 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 stitT and comparatively stable clays, the 

 skin friction of the shield is comparatively small, and an ag- 

 gregate 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 5j ft. apart, 

 and have a working diameter of from 5 to 6 ins., with a 

 water pressure of about 1,000 Ibs. 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, re- 

 sprc -lively, about 6 or 7 ins., and from 4,000 llw. to 6,000 Ibs. 

 per sq. in. With these high pressures, power pumps are 

 necessary to give the required water pressure; but where the 

 pressure required does not cxcrrd 1. <><)<) Ibs. jer sq. in., hand 

 jiii mps may l>e, ami usually are, employed. The number of 

 jacks required dejN-nds upon the diameter of the shield, and, of 

 course, ui*>n tM(> distance apart which they an- placed. In the 

 City and South London tunnel shield six jacks were used, and 



