the slides in the shutters were opened, and the material in front was
raked into the shield. At the same time, the shutters were gradually
worked forward. The two upper shutters in a compartment were generally
advanced from 12 to 15 in., after which the muck could be shoveled out
over the bottom shutters, as shown on Fig. 6, Plate LXVIII, and Fig. 3,
Plate LXX. No shutters were placed in the bottom compartments, and as
the air pressure was not generally high enough to keep the face dry at
the bottom, these compartments were pretty well filled with the soft,
wet quicksand. Just before shoving, this material was excavated to a
point where it ran in faster than it could be taken out. Much of the
excavation in the bottom compartment was done by the blow-pipe. During
the shove the material from the bottom compartment often ran back
through the open door in the transverse bulkhead, as shown by Fig. 5,
Plate LXVIII.

In the Blackwall Tunnel the material was reported to have been loose
enough to keep in close contact with the shutters at all times. In the
East River Tunnels this was not the case. The sand at the top was dry
and would often stand with a vertical face for some hours. In advancing
the shutters, it was difficult to bring them into close contact with the
face at the end of the operation. The soft material at the bottom was
constantly running into the lower compartment and undermining the stiff
dry material at the top. The latter gradually broke away, and, at times,
the actual face was some feet in advance of the shutters. Under those
circumstances, the air escaped freely through the unprotected sand face.
The joints of the shutters were plastered with clay, but this did not
keep the air from passing out through the lower compartments. This
condition facilitated the formation of blows, which were of constant
occurrence where shutters were used in the sand. In Tunnels _B_ and _D_,
at Manhattan, the shutters were used in the above manner clear across to