[Sidenote: _Typical
Subway_]

The general character of the flat roof "I" beam construction is shown
in photograph on page 28 and drawing on this page. The bottom
is of concrete. The side walls have "I" beam columns five feet apart,
between which are vertical concrete arches, the steel acting as a
support for the masonry and allowing the thickness of the walls to be
materially reduced from that necessary were nothing but concrete used.
The tops of the wall columns are connected by roof beams which are
supported by rows of steel columns between the tracks, built on
concrete and cut stone bases forming part of the floor system.
Concrete arches between 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,