the truck was 5 feet and the pairs of axles were alternately 27 and 9
feet apart. The traction load was taken at 20 per cent. of the live
load, and a wind pressure of 500 pounds per lineal foot was assumed
over the whole structure.

[Sidenote: _Tubes under
Harlem River_]

One of the most interesting sections of the work is that which
approaches and passes under the Harlem River, carrying the two tracks
of the East Side line. The War Department required a minimum depth of
20 feet in the river at low tide, which fixed the elevation of the
roof of the submerged part of the tunnel. This part of the line, 641
feet long, consists of twin single-track cast-iron cylinders 16 feet
in diameter enveloped in a large mass of concrete and lined with the
same material. The approach on either side is a double-track concrete
arched structure. The total length of the section is 1,500 feet.

The methods of construction employed were novel in subaqueous
tunneling and are partly shown on photographs on pages 62 and 63.
The bed of the Harlem River at the point of tunneling consists of mud,
silt, and sand, much of which was so nearly in a fluid condition that
it was removed by means of a jet. The maximum depth of excavation was
about 50 feet. Instead of employing the usual method of a shield and
compressed air at high pressure, a much speedier device was contrived.

The river crossing has been built in two sections. The west section
was first built, the War Department having forbidden the closing of
more than half the river at one time. A trench was dredged over the
line of the tunnel about 50 feet wide and 39 feet below low water.