rally arises
from an improper size of pillars; some roofs, however, are so difficult
to support that sits take place where the half of the coal is left in
pillars. Fig. 4 will convey a general idea of the appearance of
sits,--k, m, n showing different stages.

[Illustration: FIG. 5.--Pillar Working.]

The modern method of pillar working is shown in fig. 5. In the
Northumberland steam coal district, where it is carried out in the most
perfect manner, the bords are 5 to 6 yds. in width, while the pillars
are 22 yds. broad and 30 yds. long, which are subsequently got out on
coming back. In the same figure is also shown the method of working
whole coal and pillars at the same time, a barrier of two or three
ranges of pillars or a rib of solid coal being left between the working
in the solid and those in the pillars. The space from which the entire
quantity of coal has been removed is known in different districts as the
"goaf," "gob," or "waste."

[Illustration: FIG. 6.--Lancashire method of working Coal.]

Fig. 6 represents the Lancashire system of pillar working. The area is
laid out by two pairs of level drifts, parallel to each other, about 150
yds. apart, which are carried to the boundary. About 100 yds. back from
the boundary a communication is made between these levels, from which
other levels are driven forward, dividing the coal into ribs of about 25
or 30 yds. wide, which are then cut back by taking off the coal in
slices from the level towards the rise in breadths of about 6 yds. By
this method the whole of the coal is got backwards, the main roads being
kept in solid coal; the intermediate levels not being driven till they
are wanted, a greater amount of support is given, and the pillars are
less crushed than is usual in pillar working.

In the South Wales system of working, cross headings are driven from the
main roads obliquely across the rise to get a sufficiently easy gradient
for horse roads, and from these the stalls are opened out with a narrow
entrance, in order to leave support on either side of the road, but
afterwards widening to as great a breadth as the seam will allow,
leaving pillars of a minimum thickness. The character of such workings
is very irregular in plan, and as the ventilation is attended with
considerable difficulty, it is now becoming generally superseded by more
improved methods.

[Illustration: FIG. 7.--Long-wall method of working Coal in Derbyshire.]

Long-wall working.