iven over the area to be covered by the subway, 6 feet 4 inches
apart laterally and 8 feet longitudinally. They were cut off about 11
feet above the center line of each tube and capped with timbers 12
inches square. A thoroughly-trussed framework was then floated over
the piles and sunk on them. The trusses were spaced so as to come
between each transverse row of piles and were connected by eight
longitudinal sticks or stringers, two at the top and two at the bottom
on each side. The four at each side were just far enough apart to
allow a special tongue and grooved 12-inch sheet piling to be driven
between them. This sheathing was driven to a depth of 10 to 15 feet
below the bottom of the finished tunnel.

A well-calked roof of three courses of 12-inch timbers, separated by
2-inch plank, was then floated over the piles and sunk. It had three
timber shafts 7 x 17 feet in plan, and when it was in place and
covered with earth it formed the top of a caisson with the sheet
piling on the sides and ends, the latter being driven after the roof
was in place. The excavation below this caisson was made under air
pressure, part of the material being blown out by water jets and the
remainder removed through the airlocks in the shafts. When the
excavation was completed, the piles were temporarily braced and the
concrete and cast-iron lining put in place, the piles being cut off as
the concrete bed was laid up to them.

The second or eastern section of this crossing was carried on by a
modification of the plan just mentioned. Instead of using a temporary
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 The steel piles were withdrawn by a six-part tackle and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
as