tunnels instead of a larger tunnel containing two tracks was to avoid
the danger of accidents due to the obstruction of both tracks by
derailment or otherwise. The tunnels are made just large enough to allow
the passage of a train with perfect safety, as it was believed that with
such an arrangement thorough ventilation would be secured by the motion
of the trains. Experience seems to justify this assumption, but, in
order to assure thorough ventilation under unusual conditions, such as
the stoppage of trains in the tunnels, a complete ventilating plant will
be provided for each tunnel. The rapidity and safety of construction
were increased by making the tunnel as small as possible, one of the
difficulties in the shield method of construction being the difference
in hydrostatic pressure between the top and bottom of the shield, which
increases with the diameter of the tunnel.

The concrete lining was introduced to insure the permanency of the
structure, strengthen it from outward pressure and guard it against
injury from accidents which might occur in the tunnel. The side concrete
benches were suggested by Mr. Cassatt, President, to confine the trains
to the center of the tunnels in case of derailment, and to furnish
sidewalks on each side of the trains so as to obviate the necessity of
walking on the track.

Refuge niches are constructed in the side benches of the tunnels.
Manholes, splicing chambers, pump chambers, and other features for the
handling of the electric cables and drainage, are established at
intervals.

At points where unusual stresses were anticipated, as for instance where
the tunnels pass from rock to soft ground, the shell was composed of
steel instead of cast-iron plates. In the North River tunnels the