316 GEOLOGY OF OHIO. 
the mine in the afternoon, and during the night the weight of the over- 
lying strata breaks down the coal, which falls forward on a range of 
sprags planted for the purpose by the miners. In some mines there is 
a wall-face from three to four hundred yards in length, extended in a 
straight line, and sometimes a line of face is formed like the arc of a 
circle. | 
As by this method of mining, artificial pillars must be erected to 
prevent the closing in of the workings, consequent on the removal of 
all the coal, skill and judgment are required in building pack-walls and 
maintaining roads. The pack-walls are built on each side of the haul- 
ing roads, from the debris of the seam or from the cuttings of the roof 
or bottom. In some mines, where suitable material does not exist un- 
_derground for puilding purposes, it is brought down from the surface, 
but this adds materially to the cost of mining. 
This system of working is largely followed in Scotland, fifty-five 
per cent. of the coal of that country being mined on the long-wall plan. 
In the central counties of England the thin seams are nearly all worked 
in this manner, and in Somersetshire beds of coal only 13 inches in 
thickness are worked by long-wall. In South Wales the system is in 
high favor. In Belgium and Saxony, on the continent of Europe, long- 
wall mining is also extensively followed. 
While the long-wall system is mainly confined to seams of ordinary 
thickness, it is capable of more general application than is generally be- 
lieved. It has been successfully applied to the great thick coal of 
South Staffordshire, which ranges from 21 to 36 feet in height ; the coal 
is mined in several lifts, beginning at the top, and is worked away in 
long sweeping sides of working-courses, which range from 3 to 6 feet in 
thickness. After the first lift of workings is finished, a second or lower 
lift is opened out by driving forward a new set of galleries and opening 
out a new range of walls, and so on. 
By the long-wall system nearly all the coal mined is round or 
lump, 14 per cent. of slack being made by pillar and room-working, 
against 8 per cent. by long-wall, and the ventilation is also simple, there 
being but one main air course in the interior of the mine at the wall- 
face. In addition also to the greater acreage of coal recovered by this 
system, the expense of getting the coal is materially reduced ; but addi- 
tional care and skill are required in the management of mines. 
