srii.MAi:iNK TI NM:LIN<; 223 



was flattened 3 ins. by its own weight At the East River 

 tunnel a similar experiment was made ; two rings of plates 

 \\cre bolted together, breaking joints, one ring being revolved 

 two hull's. These two rings were then raised upright, but no 

 flattening could be detected. By means of a turnbuckle a 

 measun d strain was now brought upon the rings along the 

 vertical diameter. At 16,000 Ibs. the vertical diameter was 

 shurtened -in., the flanges of the plates cracking where the 

 turnbuckle was attached. In these two instances there was, of 

 course, a great difference in the size of the rings, those in the 

 Hudson tunnel being 18 ft inside diameter, while those in the 

 gas tunnel were only 10 ft. 2 ins. inside diameter. 



Aside from the rigidity gained, breaking joints has proved 

 much the better in other ways. With continuous joints, two 

 things are apt to occur: (1) The joint-face where two rings 

 meet may become slightly warped ; that is, all points on this 

 face of the ring will no longer lie in the same plane. This 

 may be caused by carelessness in allowing dirt to get into the 

 joints between the rings. When this once occurs the warping 

 increases with every additional ring till true joints can no 

 longer be made. (2) The rings may be erected so as to depart 

 gradually from a true circular form. This latter case is im- 

 possible where the joints are broken, and, in the former in- 

 stance, by breaking joints, the error is divided and distributed 

 around the ring until it disappears. 



On Jan. 16, 1894, the end of the soft seam was reached with 

 the shield, and ruck was again entered after having passed 

 through 08 ft. of soft ground. This rock resembled slightly 

 tin- lock on Blackwell's Island. It was in a much shattered 

 condition, \\jth many loose heads and small, soft veins. As 

 this material required support in the heading and a permanent 

 lining, and as, in its present condition, there was no assurance 

 that it might not again pass into soft material shield tunnel- 

 ing was still continued. Small machine-drills were set up in 

 the four working-chambers of the shield upon arms bolted to 



