and the U. S. Water-proofing Company's compound--have been made
with apparent success up to the present time, and the results after the
lapse of a considerable period are awaited with interest.

_Duct Laying._--The position of the electric conduits, buried in the
heart of the concrete walls, interfered greatly with the economical and
speedy placing of the lining, and their laying proved to be one of the
most troublesome features of the work. The power conduits were
single-way, with the bank for high-tension cables separated in the
side-walls from the low-tension bank, as shown on Plate XII. The
conduits for telephone and telegraph service were four-way, and were
located in the core-wall. All ducts had 3/4-in. walls and a minimum
clear opening of 3-3/8 in. square, with corners rounded. They were laid
with joints broken in all directions, and in about 1/4-in. beds of
1:2-1/2 mortar. Flat steel bond-irons, 2 by 1/8 in., with split and bent
ends, were placed in the joints at intervals of 3 ft. and projected into
the concrete 3 in. on each side, tying together the concrete on opposite
sides of the ducts. The joints were wrapped with a 6-in. strip of
10-oz. duck saturated with neat-cement grout, and, in addition, the
power conduits were completely covered with a 1/2-in. coat of mortar to
prevent the intrusion of cement and sand from the fluid concrete. The
four-way conduits were plastered only over the wraps. Splicing chambers
were provided at intervals of 400 ft.

[Illustration: PLATE LXII, FIG. 1.--WATER-PROOFING OVER BRICK
ARCHES.]

[Illustration: PLATE LXII, FIG. 2.--TRESTLE USED IN CONCRETING IN
THREE-TRACK TUNNEL.]

[Illustration: PLATE LXII, FIG. 3.--METHOD OF STREET SUPPORT OVER
OPEN-CUT EXCAVATION.]

[Illustration: PLATE LXII, FIG. 4.--JUNCTION OF TWIN AND THREE-TRACK
TUNNELS.]

THREE-TRACK TUNNEL LINING.

In the Three-Track Tunnels, a heavy brick arch was used for those
portions constructed in tunnel, while, in the open-cut sections, the
roof was of concrete. Both were completely water-proofed on the roof and
sides, and in the tunnel sections the space above the brick roof was
filled with rock packing. On account of the unstable nature of the rock
encountered throughout, the voids in the packing were afterward filled
with grout.

By reference to the cross-sections, Plate XII, it will be seen that the
haunches of the arch were tied together by steel I-beams anchored in the
concrete, with the object of ma