In the course of their progress, the shields were subjected to the most intense strains and hard usage, as may well be imagined. One of the shields is illustrated. It was used to construct the south tunnel of the up-town pair of tubes, and passed from under the Hudson River, through Morton, Greenwich and Christopher Streets, into Sixth Avenue, and north to Twelfth Street, a total distance of 4,525 feet, of which

North Tunnel, Showing Commencement of New Work.

2,075 feet was through rock overlaid with wet sand. During the progress of this shield, 26,000 sticks of dynamite were exploded in front of the cutting edge, causing great damage to the structure of the shield, so that when it arrived at its destination at Sixth Avenue and Twelfth Street, it was in such a condition of distortion that it was with difficulty that the tunnel lining could be erected behind it.

In pushing a shield forward with the battery of powerful hydraulic jacks, each advance is of two feet, and must be followed immediately by installation of the permanent lining in the rear. In the early days, brick work was used for lining, and in recent years it has also been used to some extent, but even with the use of quick-setting Portland cement, neither brick work nor concrete has proved successful for subaqueous work, as the cement cannot reach the required strength within the time it is feasible to leave the shield standing before advancing it again.

During the early work on the north tube of the uptown tunnels, a point was reached where the rock was sixteen feet above the bottom of the tunnel, and the overlying silt was in a semi-fluid state. Five barges of clay had been dumped in the met with in tunneling under a river has been solidified by means of fire. Seven days after passing this troublesome point, the rock suddenly disappeared and the work proceeded without further trouble.

Hole Broken Through the South Tube of the New York and Jersey Tunnel Looking West.

river over this point to make a roof for the tunnel, but the fluid clay could not be controlled, and crept through the doors of the shield. After trying all known methods to get through, it was decided to bake this wet clay by means of intense heat. Two large barges of kerosene were sent into the tunnel, and an air pipe connected to them. Fine blow-pipes were also attached, and the fire from the blow-pipes was impinged on the exposed clay until it became caked sufficiently dry and hard to overcome slipping. It required eight hours of this baking to dry the clay hard, and, during this period, water had to be played continuously on the shield to avoid damage due to the high temperature. It is believed that this was the first time that soft material

New York and New Jersey Tunnel Showing Signal and Car.

Another unusual situation occurred in the south tunnel of the uptown tubes. When the shield had advanced 115 feet from the Jersey side, the night superintendent in charge of the tunnel work, in his anxiety to push the work, disobeyed instructions, and the tunnel got away from him and was flooded, and his men had a narrow escape with their lives. In order to regain the tunnel, several schemes were considered, including that of sending a dredge through to dredge out the bed of the river just in advance of the shield, a sufficient depth to enable a diver to go down and timber up the exterior opening of the doorway, where the silt and mud had come through and filled the tunnel. This plan had to be abandoned, as the river above was almost entirely occupied by shipping that could not be interrupted.

Finally the difficult situation was met by obtaining two large and heavy mainsails, which made a double canvas cover measuring about sixty by forty feet. This

An X-Ray View or a Busy Half-Mile Under the Ground on the Jersey Side of the.

Hudson River

Cross-Section on Sixth Avenue at Thirty-third Street, New Fork.

1. Foot Passage

2. Manhattan Elevated Railroad

3. Street Surface and Metropolitan Street Railway

4. New Rapid Transit Subway

5. Hudson and Manhattan Railroad Station

6. Pennsylvania Railroad Tunnel canvas cover was then spread on a flat barge, small sections of pig iron being attached around the edges of it. Ropes were carried to fixed points to hold it in exact position. The barge was then withdrawn, and the canvas cover cropped to the bed of the liver, and, most fortunately, it settled over the point where the leak bad occurred, and a large number of bags of dirt were then deposited on it. An opening was then made in the bulkhead of the tunnel below, and for eight days material, under hydrostatic pressure, forced its way into the tunnel, where it was loaded on cars, and finally the canvas was drawn into the hole, stopping it up. Additional material was then deposited into the river to fill the cavity, and finally the tunnel was recovered, pumped out and work resumed. This event is of somewhat historical interest, in that the two mainsails which were used were procured from the owner of the famous American cup defender, the well-remembered "Reliance."

Probably the most unique and interesting pieces of construction are the three junctions on the Jersey side of the river, where the uptown tunnels from New York diverge, north to Hoboken and south to Jersey City and New York downtown. For safe and expeditious operation of trains, where the schedule is only one and one-half minutes, it was imperative that grade crossings should be avoided. By grade crossings is meant the tracks of one service crossing the tracks of another service at the same grade. At the point in question, this was a knotty problem to solve, owing to the unusual operating conditions which had to be met, there being six separate and distinct operating classes of trains to be handled around this triangle.

To meet this situation, three massive reinforced concrete caissons were built on the surface. They are practically large two-story houses, each being over one hundred feet in length, about fifty feet in height, and about forty-five feet in width at their widest point. The bottom edges were sharp, and, with the use of air pressure and great weights, the three structures were sunk in the ground to the same grade as the intercepting tunnels, and the tunnels were then driven into them.

Particular attention should be given to the Jersey City to Hoboken tube, in the lower part of the caisson in the foreground, in the accompanying illustration, which curls around the Hoboken to Jersey City tube, and rises to the elevation of, and connects into, the New York to Hoboken tube, at the caisson in the background, at the left of the illustration. Very few of the people who travel through the tube are probably aware of such manipulation. At the same time, the arrangement absolutely avoids any grade crossing whatever, and without such an arrangement of tracks the road could not be operated with trains run so closely together as under the prevailing system.

In constructing the river tunnels the work was carried on simultaneously from opposite sides of the river, the tunnels meeting under the river, and it is interesting, if not remarkable, when one considers the difficulties under which the engineering work had to be carried on, to note that the tunnels met with practically absolute accuracy.