hydraulic pressure of about 2,000 tons. The two shields drifting to
the center of the river from Garden Place are in water-bearing sand
and are operated under air pressure. The river tubes are on a 3.1 per
cent. grade and in the center of the river will reach the deepest
point, about 94 feet below mean high-water level.

The typical subway of reinforced concrete from Clinton Street to the
Flatbush Avenue terminus is being constructed by the method commonly
used on the Manhattan-Bronx route. From Borough Hall to the terminus
the route of the subway is directly below an elevated railway
structure, which is temporarily supported by timber bracing, having
its bearing on the street surface and the tunnel timbers. The
permanent support will be masonry piers built upon the roof of the
subway structure. Along this portion of the route are street surface
electric roads, but they are operated by overhead trolley and the
tracks are laid on ordinary ties. It has, therefore, been much less
difficult to care for them during the construction of the subway. Work
is being prosecuted on the Brooklyn Extension day and night, and in
Brooklyn the excavation is made much more rapidly by employing the
street surface trolley roads to remove the excavated material. Spur
tracks have been built and flat cars are used, much of the removal
being done at night.




CHAPTER III

POWER HOUSE BUILDING