[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 the other hand, as
the use of compressed air in building the tunnels was anticipated, an
excessive depth below the water surface was to be avoided as far as
possible; it was necessary, however, to continue the descending grade
some further distance until the tunnels were mostly in rock, so that
drainage sumps under the tunnels could be made readily. Eastward from
the sumps the tunnels had a rising grade of 0.7% to the established
bulkhead line on the Long Island side, giving a cover at the points
where the tunnels enter rock, a short distance westward, of about 10 ft.
(if the dredging plane should be fixed at some future time at 40 ft.
below mean low tide, as may be reasonably anticipated). Eastward from
the bulkhead line, Tunnels _A_, _B_, and _D_ have ascending grades of
about 1.25%, while Tunnel _C_ rises at the rate of 1.9% in order to
effect a crossing over Tunnel _B_ west of the portals. This feature was
introduced in order to place the two west-bound tracks together through
the Sunnyside Yard, and the heavier grade, being downward with the
traffic, was not objectionable.

The arrangement of grades and tracks in the approaches and in Sunnyside
Yard would require the introduction of too much detail to be taken up
here, but will be dealt with in the paper on the Sunnyside Yard.

It was recognized from the inception of the project that the tunnels
under the East River would be the most difficult and expensive section