Mines are excavations made in the earth for the purpose of raising
the various minerals which exist below its surface, such as coal,
rock-salt, and the various ores from which metals are extracted (see
“Smelting”). Mines consist of those which contain minerals that lie
in strata _parallel_ (or nearly so) to the surface of the earth, as
coal, rock-salt, or iron-stone, and those containing the ores and
minerals which are imbedded in seams or fissures of the primitive
rocks, and are nearly _perpendicular_ to the surface. Of the former
kind, coal-mines form the chief examples. When indications of coal
are discovered, a “boring” is commenced to ascertain its existence,
and the depth at which it is placed below the surface. Each piece of
earth raised by the boring-tools is placed one beside the other, in
the exact order in which they are raised, so as to show the kind of
earth being bored through, and the thickness of each strata between
the surface of the earth and the seam of coal; and it sometimes
happens that the boring is stopped on arriving at certain kinds of
rock--the old red sandstone, for example--for it would be useless
to continue boring beyond this, no coal ever existing below it.
When coal is found, and its quality and the thickness of the seam
ascertained to be such as to warrant further expense, a shaft is dug
down of some eight or ten feet diameter, cased with brickwork or wood
to prevent the falling-in of its sides, and in some cases powerful
machinery has to be erected to pump out the water which flows in.
On reaching the coal, galleries--called “gates,” or “bords”--are
dug in it in opposite directions, forming one long straight passage,
and from this other smaller ones, called “headways,” are dug, at
right angles, to the depth of about twenty-four feet, and from these
other “gates” are carried parallel with the first, forming a series
of roadways joined by short passages, and having squares of coal
between them; the height of all these passages is determined by
the thickness of the seam of coal, usually from three to ten feet.
The great masses of coal forming the squares between these passages
are gradually dug away (as far as can be done with safety) and the
gates continued onwards, but before long the ventilation becomes
impeded, and the air foul and dangerous from “fire-damp” (carburetted
hydrogen) or “choke-damp” (carbonic acid), gases which are given off
from the fissures in the coal. It is therefore necessary to produce
a continuous current of fresh air in every part of the mine, which
is done by sinking another shaft at the furthest part of the mine and
keeping a large fire burning at its mouth, over which a tall shaft is
generally erected, from which a column of light air ascends, drawing
fresh air down the other shaft and through every part of the mine,
to supply this “up-cast” shaft, as it is called. This supply of fresh
air is economised and regulated by doors or valves, so placed that
any part requiring extra ventilation can obtain it at any moment by
shutting these doors and letting the whole current go through that
particular part.
The removal of coal is effected partly by digging with the “pick,”
and partly by blasting with gunpowder; a large square mass is cut all
round, and a charge of powder fired behind it, so as to bring down at
once sixty or eighty tons of coal, which is brought along the gates
on “trams” to the bottom of the shaft, where “corves” or baskets
filled with it are drawn up to the “pit’s-mouth” by steam machinery,
one corve ascending full while another is descending empty.
The mines from which most minerals, such as sulphuret of lead
(galena) or of copper, are drawn, belong to the second class,
or those whose shafts “cut” the vein of mineral at a very acute
angle. When the existence of the required mineral and its “dip” or
inclination is ascertained, a shaft is sunk so as to cut its upper
surface, and then carried through it, cross-cuts being formed on to
the vein, and “levels” or galleries right and left in the direction
of the vein. From these levels “winzes” or small shafts are cut
at intervals from one level to that below it, thus leaving square
portions of the mineral vein to be explored, which is done by digging
away the roof or upper part, so that the rubbish and ore falls down,
when it is sorted and carried away.