ords and banks. The
field is divided by levels and headings into rectangular banks, while
from the main levels bords or wickets about 30 yds. wide, separated from
each other by banks of about the same width, are carried forward in
long-wall work, as shown on the left side of the figure, the waste being
carefully packed behind so as to secure the ventilation. When these have
been worked up to the extremity, as shown on the right side, the
intermediate bank is removed by working backward towards the level. This
system, therefore, combines both methods of long-wall working, but it is
not generally applicable, owing to the difficulty of ventilation, due
to the great length of air-way that has to be kept open around the waste
on each bank.

The relative advantages of the different methods may be generally stated
as follows. Long-wall work is best suited for thin coals, and those
having a good roof, i.e. one that gives way gradually and fills up the
excavation made by removing the coal without scaling off suddenly and
falling into the working faces, when practically the whole of the coal
may be removed. Against these advantages must be placed the difficulties
attending the maintenance of roads through the goaves, and in some cases
the large proportion of slack to round or large coal obtained. Pillar
working, in the whole coal, is generally reputed to give a more
advantageous proportion of round coal to slack, the latter being more
abundantly produced on the removal of the pillars, but as these form
only a small portion of the whole seam, the general yield is more
advantageous than in the former method. The ventilation of pillar
working is often attended with difficulty, and the coal is longer
exposed to the influence of the air, a point of importance in some
coals, which deteriorate in quality when exposed to a hot damp
atmosphere. The great increase in the size of the pillars in the best
modern collieries worked upon this principle has, however, done much to
approximate the two systems to an equality in other respects.

Where the whole of the coal is removed at once there is less chance of
surface damage, when the mines are deep, than with pillar workings. A
notable instance of this was afforded at Newstead, Notts, where the
ruined front of Newstead Abbey was lowered several feet without any
injury to the structure.

[Illustration: FIG. 8.--Bords and Banks.]

Working thick seams.

The working of very thick seams pr