ng while the former
operation was in progress. To reduce the delays, the heading was raised,
thus reducing the quantity of rock left in the top, and the bottom was
taken out as a bench. To avoid blocking the tracks when firing the top,
a heavy timber platform was built out from the floors of the middle
working compartments. Most of the muck from the top was caught on the
platform and dropped into cars below. This method of working is shown by
Fig. 2, Plate LXVII. The platforms were not entirely satisfactory, and,
later, the drills in the heading were turned upward and a top bench was
also drilled and fired, as shown by Fig. 3, Plate LXVII. There was then
so little excavation left in the top that the muck was allowed to fall
on the tracks and was quickly cleared away. The method just outlined is
called the center-heading method, and was the most satisfactory plan
devised for full-rock sections.

_Excavation in Part Rock and Part Earth._--This was probably the most
difficult work encountered, particularly when the rock was covered with
boulders and coarse sharp sand which permitted a free escape of air. It
was necessary, before removing the rock immediately under the soft
ground, to excavate the earth in advance of the shield to a point beyond
where the rock was to be disturbed, and to support, in some way, the
roof, sides, and face of the opening thus made. The hoods were designed
mainly for the purpose of supporting the roof and the sides. With the
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 tim