. base and 9 in. high is cut along
the face, inclined timber props being placed at intervals to support the
overhanging portion until the required length is cut. These are then
removed, and the coal is allowed to fall, wedges or blasting being
employed when necessary. The roof of the excavation is supported as the
coal is removed, by packing up the waste material, and by a double row
of props, 2 ft. from each other, placed temporarily along the face.
These are placed 5 ft. apart, the props of the back row alternating with
those in front. The props used are preferably of small oak or English
larch, but large quantities of fir props, cut to the right length, are
also imported from the north of Europe. As the work proceeds onwards,
the props are withdrawn and replaced in advance, except those that may
be crushed by the pressure or buried by sudden falls of the roof.

In Yorkshire hollow square pillars, formed by piling up short blocks of
wood or chocks, are often used instead of props formed of a single stem.

In securing the roof and sides of coal workings, malleable iron and
steel are now used to some extent instead of timber, although the
consumption of the latter material is extremely large. As a substitute
for timber props at the face, pieces of steel joists, with the web cut
out for a short distance on either end, with the flanges turned back to
give a square bearing surface, have been introduced. In large levels
only the cap pieces for the roof are made of steel joists, but in
smaller ones complete arches made of pieces of rails fish-jointed at the
crown are used. In another system introduced by the Mannesmann Tube
Company the prop is made up of weldless steel tubes sliding
telescopically one within the other, which are fixed at the right height
by a screw clamp capable of carrying a load of 15 to 16 tons. These can
be most advantageously used on thick seams 6 to 10 ft. or upwards. For
shaft linings steel rings of H or channel section supported by
intermediate struts are also used, and cross-bearers or buntons of steel
joists and rail guides are now generally substituted for wood.

When the coal has been under-cut for a sufficient length, the struts are
withdrawn, and the overhanging mass is allowed to fall during the time
that the workmen are out of the pit, or it may be brought down by
driving wedges, or if it be of a compact character a blast in a borehole
near the roof may be required. Sometimes, but rarely, it