[Illustration:  MOVING WEST SIDE WALL TO WIDEN SUBWAY FOR THIRD TRACK—­135TH STREET AND BROADWAY]

[Illustration:  SUBWAY THROUGH NEW “TIMES” BUILDING, SHOWING INDEPENDENT CONSTRUCTION—­THE WORKMEN STAND ON FLOOR GIRDERS OF SUBWAY]

[Illustration:  COLUMNS OF HOTEL BELMONT, PASSING THROUGH SUBWAY AT 42D STREET AND PARK AVENUE]

In a number of places it was necessary to underpin the columns of the elevated railways, and a variety of methods were adopted for the work.  A typical example of the difficulties involved was afforded at the Manhattan Railway Elevated Station at Sixth Avenue and 42d Street.  The stairways of this station were directly over the open excavation for the subway in the latter thoroughfare and were used by a large number of people.  The work was done in the same manner at each of the four corners.  Two narrow pits about 40 feet apart, were first sunk and their bottoms covered with concrete at the elevation of the floor of the subway.  A trestle was built in each pit, and on these were placed a pair of 3-foot plate girders, one on each side of the elevated column, which was midway between the trestles.  The column was then riveted to the girders and was thus held independent of its original foundations.  Other pits were then sunk under the stairway and trestles built in them to support it.  When this work was completed it was possible to carry out the remaining excavation without interfering with the elevated railway traffic.

At 64th Street and Broadway, also, the whole elevated railway had to be supported during construction.  A temporary wooden bent was used to carry the elevated structure.  The elevated columns were removed until the subway structure was completed at that point. (See photograph on page 45.)

[Illustration:  SMALL WATER MAINS BETWEEN STREET SURFACE AND SUBWAY ROOF, SUBSTITUTED FOR ONE LARGE MAIN—­125TH STREET AND LENOX AVE.]

[Illustration:  SPECIAL CONSTRUCTION OF 6-1/2-FOOT SEWER, UNDER CHATHAM SQUARE]

A feature of the construction which attracted considerable public attention while it was in progress, was the underpinning of a part of the Columbus Monument near the southwest entrance to Central Park.  This handsome memorial column has a stone shaft rising about 75 feet above the street level and weighs about 700 tons.  The rubble masonry foundation is 45 feet square and rests on a 2-foot course of concrete.  The subway passes under its east side within 3 feet of its center, thus cutting out about three-tenths of the original support.  At this place the footing was on dry sand of considerable depth, but on the other side of the monument rock rose within 3 feet of the surface.  The steep slope of the rock surface toward the subway necessitated particular care in underpinning the footings.  The work was done by first driving a tunnel 6 feet wide and 7 feet high under the monument just outside the wall line of the subway.  The tunnel was given a 2-foot bottom