Any arrangement of independent scaffolding may be employed for this
system, but that invented specially for the purpose by Mr. Frank West,
as shown in Fig. 26 of our engravings, is to be preferred. It not only
supplies the necessary scaffold, but also the necessary arrangements for
hoisting the slabs, as well as for raising the liquid concrete and
depositing it behind the slabs. It is really an independent scaffold,
and may be used wherever a light tramway of contractor's rails can be
laid, which in crowded thoroughfares would of necessity be upon a
staging erected over the footway. The under frame is carried upon two
bogie frames running upon the contractor's rail, by which means it is
enabled to turn sharp curves, a guide plate inside the inner rail being
provided at the curves for this purpose. The scaffold itself consists of
a climbing platform made to travel up or down by means of four posts
which have racks attached to their faces, and which are fixed to the
under frame and securely braced to resist racking strains. A worm
gearing, actuated by a wheel on the upper side of the scaffold, causes
the scaffold to ascend or descend. A railgrip, made to act at the curves
as well as on the straight portions of the rail by being attached to a
radial arm fixed to the under frame, assists the stability of the
scaffold where required, but the gauge of the rails is altered to render
the scaffold more or less stable according to its height. Combined with
the same machine, and traveling up and down one of the same posts used
for the scaffold, is an improved crane. Its action depends upon the
proposition in geometry that if the length of the base of a triangle be
altered, its angles, and therefore its altitude, are altered. A portion
of the vertical post up and down which the crane climbs forms the base
of a triangle, and a portion of the jib, together with the stay, forms
the remaining two sides. Hence, by causing the foot of one or the other
to travel upward, by means of the worm gearing, the upper end of the jib
is either elevated or depressed.