the grade could be maintained. It was considered desirable, owing to
this rising of the shields, to increase the weight of the cast-iron
lining, and this was done, making the weight of the completed tunnel
more nearly equal to the weight of the displaced material. The weight of
the cast-iron lining (with bolts) was increased from 9,609 to 12,127 lb.
per lin. ft. of tunnel. The weight of the finished tunnel with this
heavier iron is 31,469 lb. per lin. ft. The weight of the silt displaced
per linear foot of tunnel, at 100 lb. per cu. ft., is 41,548 lb. The
weight of the completed tunnel with the maximum train load is 42,869 lb.
per lin. ft.

The maximum progress at one face in any one month was 545 ft., working
three 8-hour shifts, and the average progress in each heading while
working three shifts was 18 ft. per 24 hours; while working one shift
with the heavier lining referred to above, the delivery of which was
slow, the average progress was 11 ft. per 24 hours.

[Illustration: FIG. 6.--15' 4" Span Twin Tunnels. Rock Roof.]

[Illustration: FIG. 7.--19' 6" Span Twin Tunnels.]

[Illustration: FIG. 8.--21' 6" Span Twin Tunnels]

[Illustration: FIG. 9.--PROPOSED SHIELD FOR SUBAQUEOUS TUNNELING GENERAL
ELEVATION]

[Illustration: FIG. 10.--RESTORATION OF PIER 72 (OLD 62) NORTH RIVER
TRANSVERSE SECTION AT CENTER OF PIER]

In order to permit the screw-piles to be put in place through the