he pierhead
line, the other just outside the bulkhead line, and for many minor
reasons the latter was preferable. The center lines of each pair of
tunnels were 37 ft. apart, and each shaft, therefore, was made to cross
both lines of a pair, the same as on Manhattan side of the river. It was
not expected, however, that the Long Island shafts could be built
conveniently or the tunnels begun from them in normal air.

The decision to make the shafts of permanent construction was based not
only on the desirability of having access to and egress from the tunnels
near the banks of the river for convenience of the workmen or exit for
passengers in case of accident, but to facilitate ventilation; these
locations divide the entire lengths of tunnels east of the station into
three parts, two of which were approximately 4,000 ft. each, and the
other about 5,500 ft. The accident risk was believed to be very small,
while much weight was given to the feature of facilitating ventilation.
Further studies have enhanced the importance attached to ventilation,
and it is now intended to provide appliances for mechanical ventilation
at all shafts. The plans of the shafts are shown on Plates X and XI. The
caissons for the shafts are of structural steel, with double walls,
filled between with concrete, including a cross-wall between and
parallel to the tunnels. All these structures were fitted for sinking
with compressed air, if that should prove necessary.

Although borings had shown that rock would be found at all the shaft
sites several feet above the tunnel level, it could not be determined in
advance of excavation whether the caissons would have to be sunk to full
depth; if sound, unfissured rock were found, the sinking could be
stopped above the tunnel level; but, if not, the caissons, in any case,
would have to be sunk far enough to permit placing a water-tight floor
below the tunnels, and the tunnels themselves begun through openings in
the side-walls of the caisson; such openings, therefore, closed by
removable bulkheads, were provided in all caissons.

[Illustration: PLATE XII.--Typical Tunnel Sections]

As already stated, the grade of 1.5% from Fifth Avenue eastward was
fixed with reference to the lowest point of the river bed in order to
give the requisite cover over the tunnels at the deepest point of the
channel on the west side of the reef, where the river bottom was about
60 ft. below mean high tide for a short distance. On