these were closed by bulkheads of steel plates backed by horizontal
steel girders. The shafts were sunk as pneumatic caissons to a depth of
78 ft. below mean high water. There have been a few caissons which were
larger and were sunk deeper than these, but most large caissons have
been for foundations, such as bridge piers, and have been stopped at or
a little below the surface of the rock. The unusual feature of the
caissons for the Long Island shaft is that they were sunk 54 ft. through
rock.

It had been hoped that the rock would prove sound enough to permit
stopping the caissons at or 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