section without air pressure. At about the same time top headings were
excavated westward from the shafts for a distance of 100 ft., and the
headings were enlarged to full size for 50 ft. The object was to avoid
damage to the shaft and interference with the river tunnel when work was
started by the contractor for the cross-town tunnel.

[Illustration: PLATE LXVI, FIG. 1.--REAR OF SHIELD SHOWING COMPLETE
FITTINGS.]

[Illustration: PLATE LXVI, FIG. 2.--SHIELD WITH LOWER PORTION OF
BULKHEAD REMOVED.]

The shields were erected on timber cradles in the shaft, and were shoved
forward to the face of the excavation. Concrete bulkheads, with the
necessary air-locks, were then built across the tunnels behind the
shields. The shields were erected before the dividing walls between the
two contracts were placed. Rings of iron tunnel lining, backed by
timbers spanning the openings on the west side, were erected temporarily
across the shafts in order to afford a bearing for the shield jacks
while shoving into the portals. The movement of the shield eastward was
continued in each tunnel for a distance of about 60 ft., and the
permanent cast-iron tunnel lining was erected as the shield advanced.
Before breaking out of rock, it was necessary to have air pressure in
the tunnels. This required the building of bulkheads with air-locks
inside the cast-iron linings just east of the portals. Before erecting
the bulkheads it was necessary to close the annular space between the
iron tunnel lining and the rock. The space at the portal was filled with
a concrete wall. After about twenty permanent rings had been erected in
each tunnel, two rings were pulled apart at the tail of the shield and a
second masonry wall or dam was built. The space between the two dams was