on the conditions controlling
their establishment were the most important elements.

_Location and Grades._--It was desirable to make the tunnels between
the bulkhead lines of the rivers as straight as possible, and it was
necessary to place them at sufficient depth below the dredging plane of
the War Department (which in the North and East Rivers is 40 and 26 ft.
below mean low water, respectively) to insure them against possible
injury from heavy anchors or sunken vessels. Furthermore, they had to
pass under the piers and bulkheads of Manhattan at a depth sufficient to
make it certain that they would not affect the stability of those
structures. Another consideration influencing the establishment of the
depth of the tunnels below the bottoms of the rivers became important as
soon as the method of construction by shields with compressed air was
adopted, namely, the necessity of providing sufficient cover to guard,
as far as possible, against blow-outs during construction.

The tunnels under the city, connecting the sub-river tunnels with the
Terminal Station, were located so as to give as favorable grades as
possible. The provision of the franchise requiring the tops of the
tunnels to be at least 19 ft. below the Street surface, which had been
suggested by the Company to permit of future subways, had no effect on
their location, as other conditions required them to be at a greater
depth.

The line extending westward from Bergen Hill had to be established so as
to give ample head-room at the numerous bridges over the railroads and
highways which it crosses.

Eastward from the East River tunnels, the grades were established so as
to rise as uniformly as possible to the level of the Sunnyside Yard.

The general features of the line, as finally adopted and constructed,
are as follows:

The maximum grade west of the Terminal Station occurs on the New York
side of the North River, and is 2% in the west-bound and 1.93% in the
east-bound tunnels. The ruling grades (for the ascending traffic) being
1.32% in the west-bound and 1.93% in the east-bound tunnels. In the
tunnels east of the Terminal Station the ruling grade is 1.5% for both
east-bound and west-bound traffic. There is, however, descending with
the traffic, a short section on a grade of 1.9 per cent. These grades
would be objectionable with steam locomotives under a heavy traffic, but
the development of the electric locomotive has rendered possible the
operation