timber roof on the side walls, the permanent iron and concrete upper
half of the tunnels was employed as a roof for the caisson. The trench
was dredged nearly to sub-grade and its sides provided with wharves as
before, running out to the completed half of the work. The permanent
foundation piles were then driven and a timber frame sunk over them to
serve as a guide for the 12-inch sheet piling around the site. Steel
pilot piles with water jets were driven in advance of the wood-sheet
piles, and if they struck any boulders the latter were drilled and
blasted. The steel piles were withdrawn by a six-part tackle and
hoisting engine, and then the wooden piles driven in their place.

When the piling was finished, a pontoon 35 feet wide, 106 feet long,
and 12 feet deep was built between the wharves, and upon a separate
platform or deck on it the upper half of the cast-iron shells were
assembled, 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