esents certain special peculiarities,
owing to the difficulties of supporting the roof in the excavated
portions, and supplying fresh air to the workings. The most typical
example of this kind of working in England is afforded by the thick coal
of South Staffordshire, which consists of a series of closely associated
coal seams, varying from 8 to 12 or 13, divided from each other by their
partings, but making together one great bed of from 25 to 40 ft. or more
in thickness. The partings together do not amount to more than 2 or 3
ft. The method of working which has been long in use is represented in
fig. 9. The main level or gate road is driven in the benches coal, or
lower part of the seam, while a smaller drift for ventilation, called an
air heading, is carried above it in one of the upper beds called the
slipper coal. From the gate road a heading called a bolt-hole is opened,
and extended into a large rectangular chamber, known as a "side of
work," large pillars being left at regular intervals, besides smaller
ones or cogs. The order in which the coal is cut is shown in the dotted
and numbered squares in the figure. The coal is first cut to the top of
the slipper coal from below, after which the upper portion is either
broken down by wedging or falls of itself. The working of these upper
portions is exceedingly dangerous, owing to the great height of the
excavations, and fatal accidents from falls of roof are in consequence
more common in South Staffordshire than in any other coalfield in this
country. The air from the down-cast shaft enters from the gate road, and
passes to the up-cast through the air heading above. About one-half of
the total coal (or less) is obtained in the first working; the roof is
then allowed to fall, and when the gob is sufficiently consolidated,
fresh roads are driven through it to obtain the ribs and pillars left
behind by a second or even, in some cases, a third working. The loss of
coal by this method is very considerable, besides great risk to life and
danger from fire. It has, therefore, been to some extent superseded by
the long-wall method, the upper half being taken at the first working,
and removed as completely as possible, working backwards from the
boundaries to the shaft. The lower half is then taken in the same
manner, after the fallen roof has become sufficiently consolidated to
allow the mine to be re-opened.

[Illustration: FIG. 9.--South Staffordshire method of working Thick