oming of importance in many European coalfields. In this the filling
material, preferably sand, is sent down from the surface through a
vertical steel pipe mixed with sufficient water to allow it to flow
freely through distributing pipes in the levels commanding the
excavations to be filled; these are closed at the bottom by screens of
boards sufficiently close to retain the packing material while allowing
the water to pass by the lower level to the pumping-engine which returns
it to the surface.

[Illustration: FIG. 10.--Long-wall working-face--Plan and Section.]

Methods of cutting coal.

The actual cutting of the coal is chiefly performed by manual labour,
the tool employed being a sharp-pointed double-armed pick, which is
nearly straight, except when required for use in hard rock, when the
arms are made with an inclination or "anchored." The terms pike, pick,
mandril and slitter are applied to the collier's pick in different
districts, the men being known as pikemen or hewers. In driving levels
it is necessary to cut grooves vertically parallel to the walls, a
process known as shearing; but the most important operation is that
known as holing or kirving, which consists in cutting a notch or groove
in the floor of the seam to a depth of about 3 ft., measured back from
the face, so as to leave the overhanging part unsupported, which then
either falls of its own accord within a few hours, or is brought down
either by driving wedges along the top, or by blasting. The process of
holing in coal is one of the severest kinds of human labour. It has to
be performed in a constrained position, and the miner lying on his side
has to cut to a much greater height, in order to get room to carry the
groove in to a sufficient depth, than is required to bring the coal
down, giving rise to a great waste in slack as compared with machine
work. This is sometimes obviated by holing in the beds below the coal,
or in any portion of a seam of inferior quality that may not be worth
working. This loss is proportionately greater in thin than in thick
seams, the same quantity being cut to waste in either case. The method
of cutting coal on the long-wall system is seen in fig. 10, representing
the working at the Shipley colliery. The coal is 40 in. thick, with a
seam of fire-clay and a roof of black shale; about 6 in. of the upper
part, known as the roof coal, not being worth working, is left behind. A
groove of triangular section of 30 in