I9I0.] DAVIES— TUNNEL CONSTRUCTION. 177 



Consequently all the plates cannot be made with radial joints, and a 

 key has to be employed which is inverted from the ordinary keystone 

 form, so that it can be placed in position from within the arch 

 instead of from without. The usual length of the key is com- 

 monly equal to the distance center to center of the bolts in 

 the circumferential joints, although often made very narrow 

 like a wedge. The rings used in the Hudson tunnels were 

 two feet long face to face, which means that every ring erected 

 represents two feet of finished tunnel. The rings used in the 

 Pennsylvania tunnels are 30 inches long. The rings are bolted 

 together at close intervals at the circumferential joints by heavy 

 steel bolts, and the segments making up a complete ring, by bolts in 

 each longitudinal joint. The number of segments or plates going 

 to make up the ring depends on convenience of handling, the usual 

 limitation being for a length not exceeding six feet. For the 

 Hudson and Manhattan tubes this worked out to nine segments, all 

 having the same length, and a key. The segments weigh approxi- 

 mately 1,200 pounds apiece, and the placing of these at any point 

 in the circle would be difficult and slow without a machine erector. 

 This is one of the most human-like machines possible. It reproduces 

 the human arm as exactly as possible. The erector (singular or 

 plural, for in a big-sized tunnel more than one erector may be 

 used simultaneously) is a machine either attached to the diaphragm 

 of the shield or entirely independent. We have used both types 

 and have invariably found the independent type most elastic and 

 convenient in operation. This machine may be operated entirely 

 by hvdraulic power, or, as is very usual, by hydraulic power for the 

 in-and-out movement of the arm and by pneumatic engine or electric 

 motor for the revolving movement. The end of the arm is fitted 

 with a simple attachment, representing a hand, which grips a plate 

 segment in the middle so that it will balance. The plate is first 

 dumped from a tunnel construction car on to the bottom portion of 

 the tail of the shield already pushed forward and ready to allow 

 the erection of a ring ; then the arm of the erector hangs vertically 

 and is attached to the plate ; the arm recedes by operation of the 

 hydraulic ram lifting the plate a few inches, then the arm, which 

 is pivoted about its center and partially counterbalanced for the 



PROC. AMER. PHIL. SOC, XLIX. I95 L, PRINTED JULY 5, I9IO. 



