_Contractor's Organization._--Tunnel operations were carried on
continuously for thirteen days out of fourteen, regular work being shut
down for repairs on alternate Sundays. When the required pressure was
more than 32 lb., four gangs of laborers were employed, each gang
working two shifts of 3 hours each, with an intermission of 3 hours
between the shifts. When the pressure was less than 32 lb., three gangs
were employed, each gang covering 8 hours, but with an 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