ds outside the iron
lining of the sump and the pump chamber were filled as completely as
possible with concrete, and then thoroughly grouted. Finally, the
concrete lining was put in place inside of the iron.

As shown by Fig. 3, the excavation of these chambers left a considerable
portion of the iron lining of the tunnels temporarily unsupported on the
lower inner quarter. To guard against distortion, a system of diagonals
and struts was placed as shown.

The floor of the pump chamber was water-proofed with felt and pitch in a
manner similar to that described for the caissons at Long Island City.
It was not possible to make the felt stick to the vertical walls with
soft pitch, which was the only kind that could be used in compressed
air, and, therefore, the surfaces were water-proofed by a wall of
asphalt brick laid in pitch melting at 60 deg. Fahr. Forms were erected on
the neat line, and the space to the rock was filled with concrete making
a so-called sand-wall similar to that commonly used for water-proofing
with felt and pitch. The bricks were then laid to a height of four or
five courses. The joints were filled with pitch instead of mortar.
Sheets of tin were then placed against the face of the wall and braced
from the concrete forms. As much pitch as possible was then slushed
between the brick and the sand-wall, after which the concrete in the
main wall was filled up to the top of the water-proofing course. The tin
was then withdrawn and the operation repeated. This method was slow and
expensive, but gave good results. Ordinary pitch could not be used on
account of the fumes, which are particularly objectionable in
compressed air. The 60 deg. pitch was slightly heated in the open air before
using.

[Illustration: FIG. 3.]

The sump and pump chamber on Tunnels _C_ and _D_ differed from the one
described only in minor details; but, being wholly constructed in rock,
presented fewer difficulties and permitted a complete envelope of
water-proofing to be placed in the top.

CONCRETE LINING.

The placing of concrete inside the iron tube was done by an organization
entirely separate from the tunneling force. A mixing plant was placed in
each of the five shafts. The stone and sand bins discharged directly
into mixers below, which, in turn, discharged into steel side-dump
concrete cars. All concrete was placed in normal air.

The first step, after the iron lining was scraped clean and washed down
and all leaks w