[Illustration: FIG. 1.--Triangulation System East River Tunnel]

The work of the East River Division at this site embraced the excavation
to the depth necessary for railroad tracks, and the building of a
retaining wall extending in 31st Street from the east side of Ninth
Avenue to the west side of Seventh Avenue, thence northward along
Seventh Avenue for a distance of 155.5 ft.; also a retaining wall in 33d
Street from the west side of Seventh Avenue to the east side of Ninth
Avenue, and thence southward along Ninth Avenue for a distance of 136.3
ft. This work was placed under contract June 21st, 1904, with the New
York Contracting and Trucking Company, and later assigned by that
company to the New York Contracting Company-Pennsylvania Terminal, and
was carried out under the direction of George C. Clarke, M. Am. Soc.
C. E., as Resident Engineer, by whom it will be described in detail.

[Illustration: PLATE IX.--Map of Portion of Manhattan Island from 23d to
40th Streets, Showing Former Topography From Map Made by Gen. Egbert L.
Viele in 1865]

The station tracks leading eastward from the station will converge under
Seventh Avenue and for some distance farther east, and pass into two
three-track tunnels, one under 32d Street and the other under 33d
Street, at the respective distances of 192 and 402 ft. from Seventh
Avenue. A typical cross-section of the three-track tunnel is shown on
Plate XII. The converging sections were considered as easterly
extensions of the station, and were not included in the East River
Division. Within a few hundred feet (Plate XIV), the tracks are reduced
to two, each passing into a single tube, the two tunnels under each
street being formed in one excavation, the distance between center lines
of tunnels being 20 ft. 4 in. This construction has been termed a twin