[Illustration:  THREE-TRACK CONCRETE ARCH—­117TH STREET AND BROADWAY]

A notable example of sewer diversion was at Canal Street, where the flow of the sewer was carried into the East River instead of into the Hudson River, permitting the sewer to be bulkheaded on the west side and continued in use.  On the east side a new main sewer was constructed to empty into the East River.  The new east-side sewer was built off the route of the subway for over a mile.  An interesting feature in the construction was the work at Chatham Square, where a 6-1/2-foot circular brick conduit was built.  The conjunction at this point 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]