ere stopped, was the placing of biats, marked _B_ on
Plate LXXIV. These were made up of a 6 by 12-in. yellow pine timber, 17
ft. long, with two short lengths of the same size spliced to its ends by
pieces of 12-in. channels, 3 ft. 9 in. long, clamped upon the sides.
These biats were placed every 5 ft. along the tunnel in rings having
side keys. Next, a floor, 13 ft. wide, was laid on the biats and two
tracks, of 30-in. gauge and 6-1/2-ft. centers, were laid upon the floor.
There were three stages in the concreting. Fig. 2, Plate LXXIV, shows
the concrete in place at the end of the first, and Fig. 3, Plate LXXIV,
at the end of the second stage. The complete arch above the bench walls
was done in the last operation.

Two 3 by 10-in. soldiers (_SS_ in Figs. 1 and 2, Plate LXXIV) were
fastened to each biat and braced across by two horizontal and two
diagonal braces. To each pair of soldiers a floor template, _T_, was
then nailed. The form for the center drain was then suspended as shown
in Fig. 1, Plate LXXIV. Three pieces of shuttering, _FFF_, 20 ft. long,
were then nailed to the bottom of the soldiers. One is all that would
have been needed for the first concrete placed, but it was easier to
place them at this stage than later, when there was less room. Three
rough shutters were also nailed to the curved portion for the floor
template. Opposite each biat, a bracket, _bb_, was then nailed, which
carries a set of rough boards which formed the risers for the duct
steps. Everything was then ready for concreting except that, where
refuge niches occurred, a form for the portion of the niche below the
seat was nailed to the shuttering. This form is shown at _R_ in Fig. 1,
Plate LXXIV.

[Illustration: PLATE LXXIV]

The concrete was dumped down on each side from side-dump cars standing
on the track, and, falling between the risers for the duct steps, ran or
was shoveled under the forms and down into the bottom. The horizontal
surface on each side the center drain was smoothed off with a shovel.
The workmen became very skillful at this, and got a fairly smooth
surface. This concrete was usually placed in lengths of 45 or 60 ft.
After setting for about 24 hours, the brackets, _bb_, were removed,
together with the shuttering on the steps. The triangular pieces, _t_ in
Fig. 1, Plate LXXIV, were not removed until later. Instead, a board was
laid upon this lower step on which the duct layers could work. This and
the triangular piece