sembled, their ends closed by steel-plate diaphragms and the whole
covered with concrete. The pontoon was then submerged several feet,
parted at its center, and each half drawn out endwise from beneath the
floating top of the tunnel. The latter was then loaded and carefully
sunk into place, the connection with the shore section being made by
a diver, who entered the roof through a special opening. When it was
finally in place, men entered through the shore section and cut away
the wood bottom, thus completing the caisson so that work could
proceed below it as before. Three of these caissons were required to
complete the east end of the crossing.

[Illustration: LOOKING UP BROADWAY FROM TRINITY CHURCH--SHOWING
WORKING PLATFORM AND GAS MAINS TEMPORARILY SUPPORTED OVERHEAD]

The construction of the approaches to the tunnel was carried out
between heavy sheet piling. The excavation was over 40 feet deep in
places and very wet, and the success of the work was largely due to
the care taken in driving the 12-inch sheet piling.

[Sidenote: _Methods of
Construction
Brooklyn
Extension_]

A number of interesting features should be noted in the methods of
construction adopted on the Brooklyn Extension.

The types of construction on the Brooklyn Extension have already been
spoken of. They are (1) typical flat-roof steel beam subway from the
Post-office, Manhattan, to Bowling Green; (2) reinforced concrete
typical subway in Battery Park, Manhattan, and from Clinton Street to
the terminus, in Brooklyn; (3) two single track cast-iron-lined
tubular tunnels from Battery Park, under the East River, and under
Joralemon Street to Clinton Street, Brooklyn.

Under Broadway, Manhattan, the work is through sand, the vehicular
and electric street car traffic, the network of subsurface structures,
and the high buildings making this one of the most difficult portions
of the road to build. The street traffic is so great that it was
decided that during the daytime the surface of the street should be
maintained in a condition suitable for ordinary traffic. This was
accomplished by making openings in the sidewalk near the curb, at two
points, and erecting temporary working platforms over the street 16
feet from the surface. The excavations are made by the ordinary drift
and tunnel method. The excavated material is hoisted from the openings
to the platforms and passed through chutes to wagons. On the street
surface, over and in advance of the