were two of these safety curtains in each tunnel; when one was being moved ahead nearer the shield, the other one was in place.

Behind each shield there was a traveling stage which was anchored

to the shield and had an elevator at the rear end for taking up cement and sand for grouting to the upper platform.

Fig. 10 shows the general outline of the traveling stage attached

to the shield by a turn-buckle and traveling with rollers on rails supported by steel brackets attached to the tunnel lining.

The haulage gear, which was driven by electric motors, consisted of an endless rope trailing along the floor with a few supporting rollers. Several designs of grips for attaching the ropes to the cars were tried, but they were abandoned in favor of a light hook chain.

For concreting the tunnel inexpensive electric locomotives of 711⁄2 H.P. were used, the motor pinion geared into a countershaft with a

sprocket wheel and chain drive to each axle; an overhead wire was used.

Fig. 11 shows the back view of the shield with 27 jackets, each of 9 inches diameter. One shield completely mounted weighed about 300 tons. The erectors, of which there were two in each shield, were operated with 1000 pounds hydraulic pressure. By means of a hydraulic rack on the center pinion the arm was rotated, and on the

arm was a push-and-pull cylinder with which it was possible to extend the arm so that it could reach the circumference of the tunnel.

The shields were designed first all like the Blackwall tunnel shield (see Fig. 12), with the idea that all manner of difficulties could be encountered and overcome. Double diaphragms and air-locks

through them were provided in order that a differential air-pressure could be used, so that only the men in the face would have to go into high pressure. This scheme might work well in stiff clay, but it could not be applied in a rock tunnel on account of the rough opening in the rock and the difficulty of packing tightly around the shield.

Fig. 13 shows the face of the shield with the extension hood three feet beyond the cutting-edge. The eight under-river shields were built by the New York Shipbuilding Company at Camden. This company also built one of lighter design for the work at East Avenue. There were two horizontal floors and two vertical girders in each shield, dividing it up into nine pockets. The small lock-doors and

shoots were a great handicap in passing rock excavation through-so much so that they were very much enlarged after a little experience. On the back of each shield was a safety hood that covered the opening into the upper floor, and was designed so that if the water came into the tunnel, it could not rise higher than the under side of this hood, so long as the annular space between the cast-iron lining and the tail of the shield was kept packed with clay or bags. This

was equivalent to having one of the safety curtains right up at the back of the shield.

Fig. 14 shows method of erecting the segments on the upper quarter. The segments were attached to the erector bar by a gadget with a large nut on one plain screwed end, and a flanged spigot on the other; the nut was run back far enough to insert the gadget, and the nut was spun by hand very quickly to lock it in place. This is a very

effective way of lifting the segments and saved the expense of castiron lugs on each segment. The cast iron in the four tunnels represents about 100,000 tons; each segment weighed about one ton, so that about 100,000 lugs were saved.

Fig. 15 shows the method of erecting lining in the lower quarter. Fig. 16 shows the method adopted at East Avenue, where there was no shield. The erector was carried on a platform driven by a