any scheme on which they might desire to bid, but, of course, the
decision as to the practicability of such plans rested with the Board.

Inasmuch as the shield method of construction was required, the writer
designed a shield for use in the North River Tunnels. The shield was
about 18 ft. long, over all, and was provided with a rigid but removable
hood extending beyond the normal line of the cutting edge, for use in
sand, gravel, and ballast, to be removed when the shield reached the
silt. The shields were thrust forward by twenty-four rams capable of
exerting a pressure of 3,400 tons at a hydraulic pressure of 5,000 lb.
per sq. in. Taking into account 30 lb. air pressure, this pressure was
increased to 4,400 tons. The shield was fitted with a single hydraulic
erector and hydraulic sliding platforms, and when complete weighed 194
tons. Fig. 9 is a back elevation and section of the shield.

The contract for the river tunnels was let to the O'Rourke Engineering
Construction Company on May 2d, 1904.

The shields were built in accordance with the design previously referred
to, and proved entirely satisfactory. Generally, the materials passed
through were as follows: Starting out in full face rock, from it into a
mixed face of rock and sand, thence into sand and gravel, full face of
sand, piles, rip-rap, and the Hudson silt; and all were fully charged
with water.

Compressed air, at an average gauge pressure of about 25 lb. and a
maximum of 40 lb. per sq. in., was used in the tunnels from the time the
shields emerged from full rock face until the tunnel lining had been
joined up and all caulking and grummeting had been done.