's Island
Reef was approached, and, at the end of the period, the rock surface was
within 3 ft. of the top of the shield; in Tunnel _B_, the rock of the
reef was still a little below the shield, but the overlying material
contained a large proportion of clay and held air very well. Tunnel _C_
was still in open material, but, with two lines safe and with the
increased air plant, it was deemed best to resume work in Tunnel _A_,
which was done on October 23d, 1906. Thenceforward work was continuous
in all headings until the meeting points with the Long Island shields
were reached.

This period, January to October, 1906, inclusive, was the most strenuous
of the entire work, particularly the first six months. With one and, at
times, two tunnels closed down, the consumption of air in the headings
from Manhattan was an average of more than 20,000 cu. ft. per min. for
periods of from 30 to 60 days; it was often more than 25,000 cu. ft. for
24 hours, with a maximum of nearly 29,000 cu. ft., and doubtless this
was exceeded considerably for shorter periods. On several occasions the
quantity supplied to a single tunnel averaged more than 15,000 cu. ft.
per min. for 24 hours. The greatest averages for 24 hours were obtained
later in Tunnel _A_, after the resumption of work there, and exceeded
19,000 cu. ft., but the conditions in the headings of the other lines
were then so favorable that the work was carried on continuously in all.

The deficiency in the original plant at Manhattan was so marked, and the
need of driving all headings from Long Island simultaneously so clear,
that it was decided to increase the rated capacity of the Long Island
compressor plant to 45,400 cu. ft. of free air per minute, which was
10,400 cu. ft. greater than the capacity of the Manhattan plant after
the latter had been augmented.

[Illustration: PLATE LXXII]

The earth encountered on emerging from rock, when driving westward from
Long Island, was far more compact and less permeable to air than on the
Manhattan side, but for a distance of from 400 to 600 ft. immediately
east of the reef, it was a clean open sand, and, while the shields were
passing through this, the quantity of air supplied to the four headings
seldom fell below 20,000 cu. ft. per min.; it was usually more than
25,000 cu. ft., with a recorded maximum of 33,400 cu. ft. Although this
was greater than ever used on the Manhattan side, it was more uniformly
distributed among the several