int of numerous electric surface car lines, elevated railroad
pillars, and enormous vehicular street traffic, made it imperative
that the surface of the street should not be disturbed, and the sewer
was built by tunneling. This tunneling was through very fine running
sand and the section to be excavated was small. To meet these
conditions a novel method of construction was used. Interlocked
poling boards were employed to support the roof and were driven by
lever jacks, somewhat as a shield is driven in the shield system of
tunneling. The forward ends of the poling boards were supported by a
cantilever beam. The sides and front of the excavation were supported
by lagging boards laid flat against and over strips of canvas, which
were rolled down as the excavation progressed. The sewer was completed
and lined in lengths of from 1 foot to 4-1/2 feet, and at the maximum
rate of work about 12 feet of sewer were finished per week.

[Illustration: CONSTRUCTION OF FORT GEORGE TUNNEL]

At 110th Street and Lenox Avenue a 6-1/2-foot circular brick sewer
intersected the line of the subway at a level which necessitated its
removal or subdivision. The latter expedient was adopted, and three
42-inch cast-iron pipes were passed under the subway. (See photograph
on page 50.) At 149th Street and Railroad Avenue a sewer had to be
lowered below tide level in order to cross under the subway. To do
this two permanent inverted siphons were built of 48-inch cast-iron
pipe. Two were built in order that one might be used, while the other
could be shut off for cleaning, and they have proved very
satisfactory. This was the only instance where siphons were used. In
this connection it is worthy of note that the general changes referred
to gave to the city much better sewers as substitutes for the old
ones.

A number of interesting methods of providing for subsurface structures
are shown in photographs pages 51 to 54. From the General
Post-office at Park Row to 28th Street, just below the surface, there
is a system of pneumatic mail tubes for postal delivery. Of course,
absolutely no change in alignment could be permitted while these tubes
were in use carrying mail. It was necessary, therefore, to support
them very carefully. The slightest deviation in alignment would have
stopped the service.

[Illustration: TWO COLUMN BENT VIADUCT]

[Illustration: TRAVELER FOR ERECTING FORMS, CENTRAL PARK TUNNEL--(IN
THIS TUNNEL DUCTS ARE BUILT IN THE SIDEWALL