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'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