n the roof beams complete the top of the subway.
Such a structure is not impervious, and hence, there has been laid
behind the side walls, under the floor and over the roof a course of
two to eight thicknesses of felt, each washed with hot asphalt as
laid. In addition to this precaution against dampness, in three
sections of the subway (viz.: on Elm Street between Pearl and Grand
Streets, and on the approaches to the Harlem River tunnel, and on the
Battery Park Loop) the felt waterproofing has been made more effective
by one or two courses of hard-burned brick laid in hot asphalt, after
the manner sometimes employed in constructing the linings of
reservoirs of waterworks.

[Illustration: TYPICAL SECTION OF FOUR TRACK SUBWAY]

[Illustration: FOUR-TRACK SUBWAY--SHOWING CROSS-OVER SOUTH OF 18TH
STREET STATION]

In front of the waterproofing, immediately behind the steel columns,
are the systems of terra-cotta ducts in which the electric cables are
placed. The cables can be reached by means of manholes every 200 to
450 feet, which open into the subway and also into the street. The
number of these ducts ranges from 128 down to 32, and they are
connected with the main power station at 58th and 59th Streets and the
Hudson River by a 128-duct subway under the former street.

[Sidenote: _Reinforced
Concrete
Construction_]

The reinforced concrete construction substitutes for the steel roof
beams, steel rods, approximating 1-1/4 inches square, laid in varying
distances according to the different roof loads, from six to ten
inches apart. Rods 1-1/8 inches in diameter tie the side walls,
passing through angle columns in the walls and the bulb-angle columns
in the center. Layers of concrete are laid over the roof rods to a
thickness of from eighteen to thirty inches, and carried two inches
below the rods, imbedding them. For the sides similar square rods and
concrete are used and angle columns five feet apart. The concrete of
the side walls is from fifteen to eighteen inches thick. This type is
shown by photographs on page 41. The rods used are of both square
and twisted form.

[Illustration: LAYING SHEET WATERPROOFING IN BOTTOM]

[Illustration: SPECIAL BRICK AND ASPHALT WATERPROOFING]

[Sidenote: _Methods of
Construction
Typical
Subway_]

The construction of the typical subway has been carried on by a great
variety of methods, partly adopted on account of the conditions under
which the work had to be prosecuted and