the three-track construction was reconsidered, and two
important changes were made in the plans. The first of these was to
continue the twin tunnel westward to Sixth Avenue in 32d Street, and to
a point 180 ft. west of Sixth Avenue in 33d Street; the twin tunnel
being 9-1/2 ft. less in height than the three-track tunnel and 9 ft.
narrower, the change reduced the difficulties considerably. Where the
three-track tunnel was thus eliminated, there was no longer objection to
a steeper grade, so that, going eastward from the station, a grade of
0.8% in 33d Street and 0.9% in 32d Street was substituted for the
original 0.4% grade. From the west line of Fifth Avenue eastward short
sections with descending grades of 0.3% connect with the original 1.5%
grade near Madison Avenue. The effect of these two changes--type of
tunnel and grade--was to lower the roof of the tunnels at Fifth Avenue
about 15 ft., which made it practicable to avoid open cutting east of
Sixth Avenue.

A full account of the construction of the cross-town tunnels will be
given by the Resident Engineers.

Permanent shafts were made on both sides of the East River, those in
Manhattan being located a few feet east of First Avenue, and those in
Long Island City being located, one in the so-called Annex Slip, the
other in the pier just south of it. The two railroad lines coming from
32d Street in Manhattan, and curving to the left at Second Avenue, are
about 34 ft. apart between centers at First Avenue, and it was
convenient to make the shaft large enough to cover both lines. Borings
had shown that the excavation for the tunnels would break out of the
rock about 200 ft. east of First Avenue. It was desirable to carry the
tunnel excavation eastward from the shaft in normal air far enough to
permit of building at least 50 ft. of tunnel and installing air-locks,
so that compressed air might be available when the rock surface was
broken through. The location adopted, and shown on Plate XIII, had the
further advantages that the rock surface was several feet above the
level of the top of the tunnels, and access to the river for receiving
and discharging materials could be had without crossing any street.
Similar reasons governed the location of the north shaft for the lines
from 33d Street. On the Long Island side of the river there were only
two feasible locations meeting these conditions, particularly in respect
to a safe thickness of rock above the tunnels, one near t