fixed hood it was necessary either to excavate for the distance of the
desired shove in front of it or else to force the hood into the
undisturbed material. To avoid this difficulty, the sliding hoods were
tried as an experiment.

In using the sliding hood, which will be described in detail in Mr.
Japp's paper, the segments commencing at the top were forced forward by
the screw rod, one at a time, as far as possible into the undisturbed
material. Just enough material was then removed from underneath and in
front of the section to free it, and it was again forced forward. These
operations were repeated until the section had been extended far enough
for a shove. As soon as two or three sections had been pushed forward in
this way, the face near the advance end of the sliding hood was
protected by a breast board set on edge and braced from the face.
Gradually, all the segments were worked forward, and, at the same time,
the whole soft ground face was sheeted with timber. At times polings
were placed over the extended segments in order to make room for a
second shove, as shown on Plate LXVIII. When the shield was advanced the
nuts on the screw rods were loosened and the sections of the hoods were
telescoped on to the shield. The idea was ingenious, but proved
impracticable, because of the unequal relative movements of the top and
bottom of the shield in shoving, bringing transverse strains on the hood
sections.

[Illustration: PLATE LXIX]

With the fixed hood, poling boards were used to support the roof and
sides, and the face was supported in the manner described for the
sliding hoods. The polings were usually maple or oak planks, 2 in.
thick, about 8 in. wide, and 6-1/2 ft. long. In advancing the face, the