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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 SIDEWALLS)] [Sidenote: _Concrete-lined Tunnel_] Between 33d Street and 42d Street under Park Avenue, between 116th Street and 120th Street under Broadway, between 157th Street and Fort George under Broadway and Eleventh Avenue (the second longest double-track rock tunnel in the United States, the Hoosac tunnel being the only one of greater length), and between 104th Street and Broadway under Central Park to Lenox Avenue, the road is in rock tunnel lined with concrete. From 116th Street to 120th Street the tunnel is 37-1/2 feet