r a little below the surface and continuing
the excavation without sinking them further; for this reason only the
steel for the lower 40 ft. of the caissons was ordered at first.

The roof of the working chamber was placed 7 ft. above the cutting edge.
It was a steel floor, designed by the contractors, and consisted of
five steel girders, 6 ft. deep, 29 ft. long, and spaced at 5-ft.
centers. Between were plates curved upward to a radius of 4 ft. Each
working chamber had two shafts, 3 ft. by 5 ft. in cross-section, with a
diaphragm dividing it into two passages, the smaller for men and the
larger for muck buckets. On top of these shafts were Moran locks.
Mounted on top of the caisson was a 5-ton Wilson crane, which would
reach each shaft and also the muck cars standing on tracks on the ground
level beside the caissons. Circular steel buckets, 2 ft. 6 in. in
diameter and 3 ft. high, were used for handling all muck. These were
taken from the bottom of the working chamber, dumped in cars, and
returned to the bottom without unhooking. Work was carried on by three
8-hour shifts per day. The earth excavation was done at the rate of
about 67 cu. yd. per day from one caisson. The rock excavation,
amounting to about 6,200 cu. yd. in each caisson, was done at the rate
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
sufficien