of about 44.5 cu. yd. per day. The average rate of lowering, when the
cutting edge of the south caisson was passing through earth, was 0.7 ft.
per day. In rock, the rate was 0.48 ft. per day in the south caisson,
and 0.39 ft. per day in the north caisson.

At the beginning all lowering was done with sixteen hydraulic jacks.
Temporary brackets were fastened to the outside of the caisson. A
100-ton hydraulic jack was placed under each alternate bracket and under
each of the others there was blocking. The jacks were connected to a
high-pressure pump in the power-house. As the jacks lifted the caisson,
the blocking was set for a lower position, to which the caisson settled
as the jacks were exhausted. After the caisson had penetrated the earth
about 10 ft., the outside brackets were removed and the lowering was
regulated by blocking placed under brackets in the working chamber. The
caisson usually rested on three sets of blockings on each side and two
on each end. The blocking was about 4 ft. inside the cutting edge. In
the rock, as the cutting edge was cleared for a lowering of about 2 ft.,
6 by 8-in. oak posts were placed under the cutting-edge angle. When a
sufficient number of posts had been placed, the blocking on which the
caisson had rested was knocked or blasted out, and the rock underneath
was excavated. The blocking was then re-set at a lower elevation. The
posts under the cutting edge were then chopped part way through and the
air pressure was lowered about 10 lb., which increased the net weight to
more than 4,000,000 lb. The posts then gradually crushed and the
caissons settled to the new blocking. The tilt or level of the caisson
was controlled by chopping the posts more on the side which was desired
to move first.

The caisson nearly always carried a very large net weight, usually about
870 tons. The concrete in the walls, which was added as the caisson was