the section needing repair.

The floor of the power house between the column bases is a continuous
mass of concrete nowhere less than two feet thick. The massive
concrete foundations for the reciprocating engines contain each 1,400
yards of concrete above mean high water level, and in some cases have
twice as much below that point. The total amount of concrete in the
foundations of the finished power house is about 80,000 yards.

[Illustration: CROSS-SECTION OF POWER HOUSE]

Water for condensing purposes is drawn from the river and discharged
into it through two monolithic concrete tunnels parallel to the axis
of the building. The intake conduit has an oval interior, 10 x 8-1/2
feet in size, and a rectangular exterior cross-section; the outflow
tunnel has a horseshoe-shape cross-section and is built on top of the
intake tunnel. These tunnels were built throughout in open trench,
which, at the shore end, was excavated in solid rock. At the river end
the excavation was, at some places, almost entirely through the fill
and mud and was made in a cofferdam composed chiefly of sheet piles.
As it was impossible to drive these piles across the old timber crib
which formed the old dock front, the latter was cut through by a
pneumatic caisson of wooden-stave construction, which formed part of
one side of the cofferdam. At the river end of the cofferdam the rock
was so deep that the concrete could not be carried down to its
surface, and the tunnel section was built on a foundation of piles
driven to the rock and cut off by a steam saw 19-1/2 feet below mean
hightide. This section of the tunnel was built in a 65 x 48-foot
floating caisson 24 feet deep. The concrete was rammed in it around
the moulds and the sides were braced as it sunk. After the tunnel