concrete lining in the invert and in the arch was reinforced by
longitudinal steel bars, but these were not introduced in the East River
tunnels.

Other details connected with the structures, including the drainage,
lighting, ventilation, signaling, and electrification systems, will be
given in succeeding papers.

_Stability of the Sub-River Tunnels._--One of the most important
questions connected with the design of these tunnels was their probable
stability under the long-continued action of a heavy and rapid railroad
traffic. The tunnels are lighter than the materials which they displace
even when the weight of the heavy live load is included. In the East
River the character of the material seemed to justify the conclusion
that the tunnels would not be displaced even under the action of the
live load. In the North River, however, the tunnels are enveloped by a
soft silt and it was at first apprehended that some system of supports
would be advisable to carry the heavy traffic and insure the tunnels
against displacement under its action. To meet this contingency, which
was then believed to be a very serious one, it was proposed to sink
cast-iron screw-piles through the bottom of each tunnel into and through
the underlying silt until satisfactory bearing material was reached. The
pile supports were worked out in sufficient detail to be embraced in the
contract for the construction of these tunnels, with provision, however,
for omitting them should it be determined subsequently that their use
was undesirable. The contract plans contained provisions for sliding
joints where the piles pass through the tunnel floor, so that the live
load might be carried directly to the pile heads by a system of girders,
and also for attaching the piles directly to the tunnel, the two plans
being alternatives.