n intermission of
about 1/2 hour in low pressure for lunch.

_Air Pressures Required._--During the greater portion of the work in
soft ground, pressure was maintained which would about balance the
hydrostatic head at the axis of the tunnel. This required a pressure
varying from 30 to 34 lb. per sq. in. above that of the atmosphere. In
Tunnels _B_ and _D_, at Manhattan, during the work in soft ground,
pressures as high as 37 lb. were maintained for considerable periods of
time; in the firm material near the reef 28 lb. was often sufficient.
While removing the broken plates, the pressure was raised for a short
time to 42 lb., and was maintained between 37-1/2 and 40 lb. for a
little more than one month.

_Air Supply._--For regular operation the contractor furnished four
compressors on each side of the river, each having a rated capacity of
5,000 cu. ft. of free air per minute delivered at 50 lb. above normal,
when running at the rate of 100 rev. per min. An additional compressor
of the same capacity was supplied on each side of the river, in
compliance with the requirement for 25% excess capacity; the additional
compressors had also high-pressure air cylinders which could be
connected at will, and in which the pressure could be increased to 150
lb., and the air used to supply rock drills, grouting machines, etc. The
entire combination on each side of the river, therefore, was rated at
25,000 cu. ft. of free air per minute, or a mean of 6,250 cu. ft. per
heading. Its safe working capacity was not far from 20,000 cu. ft. per
min.

The shields broke through rock surface in Tunnels _B_, _C_, and _D_, at
Manhattan, in November and December, 1905. The consumption of air in the
four tunnels soon exceeded 15,000 cu. ft. for 24 hours, and in Tunnel
_D_, on several occasions, it exceeded 7,000 cu. ft. for a like period.
Blows had become frequent, and it was evident that the air plant was
inadequate for driving four tunnels at once in the open material east of
the Manhattan rock. Work in Tunnel _A_, therefore, was not resumed,
after the suspension on December 29th, for about ten months, and Tunnel
_C_ was also closed down for more than four months of the time between
December, 1905, and July, 1906. During this period the capacity of the
plant was increased from the rated 25,000 cu. ft. of free air per
minute, to 35,000. In Tunnel _D_ the material had gradually become
firmer, with more clay and less escape of air, as the Blackwell