aft, in position to be
shoved out of the opening in the west side of the caisson. Temporary
rings of iron lining were erected across the shaft in order to furnish
something for the shield jacks to shove against.

The roof of the working chamber was then re-erected about 35 ft. above
its original position and about 8 ft. above the tunnel openings. This
time, instead of the two small shafts which were in use during the
sinking of the caisson, a large steel shaft with a T-head lock was
built. This is illustrated in Fig. 2, Plate LXIV. The shaft was 8 ft. in
diameter. Inside there was a ladder and an elevator cage for lowering
and hoisting men and the standard 1-yd. tunnel cars. At the top, forming
the head of the T, there were two standard tunnel locks.

MANHATTAN SHAFTS.

A permanent shaft, similar to the river shafts in Long Island City, was
constructed at Manhattan over each pair of tunnels. Each shaft was
located across two lines, with its longer axis transverse to the
tunnels. Plate XIII shows their relative positions. They were divided
equally by a reinforced concrete partition wall transverse to the line
of the tunnels. On completion, the western portions were turned over to
the contractor for the cross-town tunnels for his exclusive use.

_South Shaft._--Work on the south shaft was started on June 9th, 1904,
with the sinking of a 16 by 16-ft. test pit in the center of the south
half of the south shaft, which reached disintegrated rock at a depth of
about 20 ft.

Starting in August, the full shaft area, 74 by 40 ft., was taken out in
an open untimbered cut to the rock, and a 20 by 50-ft. shaft was sunk
through the rock to tunnel grade, leaving a 10 or 12-ft. berm around it.
(Fig. 1, Plate LXX.)

The erection of the caisson was started, about the middle of January, on
the rock berm surrounding the 20 by 50-ft. shaft and about 15 ft. below
the surface. Fig. 3, Plate LXIV, shows the cutting edge of the caisson
assembled. The excavation of the small shaft had shown that hard rock
and only a very small quantity of water would be encountered, and that
the caisson need be sunk only a short distance below the rock surface.
Therefore, no working-chamber roof was provided, the caisson was built
to a height of only 40 ft., and the circular openings were permanently
closed.

The assembling of the caisson took 2-1/2 months, and on April 2d
lowering was started. Inverted brackets were bolted temporarily to the
cutting-