chutes or doors. By drilling from the shield, some muck was blasted on
to the extensions of the floors and could be handled from the upper
compartments. At best, however, the shield with the closed transverse
bulkhead was a serious obstacle to rapid work in rock sections.

The full-face method was only used where the rock was not considered
safe for a heading. A cut was fired at the bottom, together with side
holes, in a manner quite similar to that adopted in the first set of
holes for a bottom heading. The cradle was then placed, in lengths of
either 2.5 or 5 ft., after which the remainder of the face was fired in
the same manner as for the bottom-heading method. The closed transverse
bulkhead with air-locks, as shown in Fig. 1, Plate LXVI, was placed in
the shield in the hope that it would only be necessary to maintain the
full air pressure in the working compartments in front of the bulkhead.
It was also thought that some form of bulkhead which could be closed
quickly and tightly would be necessary to prevent flooding the tunnel in
case of blows. While no attempt was ever made to reduce the pressure
behind the shield bulkhead, it was obvious from the experience with
Tunnels _B_ and _D,_ while working in the sand between Manhattan and the
reef, that the plan was not practicable, and that the closed bulkhead in
the bottom was a hindrance instead of a safeguard. As soon as rock was
encountered in those tunnels at the west edge of the reef, the
contractor cut through the bulkheads and altered them, as shown in Fig.
2, Plate LXVI.

Taking advantage of the experience gained, openings were cut through the
bulkheads in Shields _A_ and _C_, while they were shut down near the
edge of the Manhattan ledge. In erecting the shields at Long Island
City in May and June, 1906, openings were also provided. These shields
had to pass through about 700 ft. of rock at the start, the greater