A sledge or mallet _fig._ 703.
Borer -- 704.
Claying bar -- 705.
Needle or nail -- 706.
Scraper -- 707.
Tamping bar -- 708.
Besides these tools the miner requires a powder-horn, rushes to be
filled with gunpowder, tin cartridges for occasional use in wet ground,
and paper rubbed over with gunpowder or grease, for the _smifts_ or
fuses.
The _borer_, _fig._ 704., is an iron bar tipped with steel, formed like
a thick chisel, and is used by one man holding it straight in the hole
with constant rotation on its axis, while another strikes the head of it
with the iron sledge or mallet, _fig._ 703. The hole is cleared out from
time to time by the scraper, _fig._ 707., which is a flat iron rod
turned up at one end. If the ground be very wet, and the hole gets full
of mud, it is cleaned out by a stick bent at the end into a fibrous
brush, called a _swab-stick_.
[Illustration: 709]
_Fig._ 709. represents the plan of blasting the rock, and a section of a
hole ready for firing. The hole must be rendered as dry as possible,
which is effected very simply by filling it partly with tenacious clay,
and then driving into it a tapering iron rod, which nearly fills its
calibre, called the _claying bar_. This being forced in with great
violence, condenses the clay into all the crevices of the rock, and
secures the dryness of the hole. Should this plan fail, recourse is had
to tin cartridges furnished with a stem or tube (see _fig._ 710.,)
through which the powder may be inflamed. When the hole is dry, and the
charge of powder introduced, the _nail_, a small taper rod of copper, is
inserted so as to reach the bottom of the hole, which is now ready for
_tamping_. By this difficult and dangerous process, the gunpowder is
confined, and the disruptive effect produced. Different substances are
employed for _tamping_, or cramming the hole, the most usual one being
any soft species of rock free from siliceous or flinty particles. Small
quantities of it only are introduced at a time, and rammed very hard by
the _tamping-bar_, which is held steadily by one man, and struck with a
sledge by another. The hole being thus filled, the nail is withdrawn by
putting a bar through its eye, and striking it upwards. Thus a small
perforation or vent is left for the rush which communicates the fire.
Besides the improved tamping-bar faced with hard copper, other
contrivances have been resorted to for diminishing the risk of those
dreadful accidents that frequently occur in this operation. Dry sand is
sometimes used as a tamping material, but there are many rocks for the
blasting of which it is ineffective. Tough clay will answer better in
several situations.
For conveying the fire, the large and long green rushes which grow in
marshy ground are selected. A slit is made in one side of the rush,
along which the sharp end of a bit of stick is drawn, so as to extract
the pith, when the skin of the rush closes again by its own elasticity.
This tube is filled up with gunpowder, dropped into the vent-hole, and
made steady with a bit of clay. A paper _smift_, adjusted to burn a
proper time, is then fixed to the top of the rush-tube, and kindled,
when the men of the mine retire to a safe distance.
In _fig._ 709. the portion of the rock which would be dislodged by the
explosion, is that included between A and B. The charge of powder is
represented by the white part which fills the hole up to C; from which
point to the top, the hole is filled with _tamping_. The _smift_ is
shewn at D.
[Illustration: 711]
_Fig._ 711. is an iron bucket, or as it is called in Cornwall, a kibble,
in which the ore is raised in the shafts, by machines called _whims_,
worked by horses. The best kibbles are made of sheet-iron, and hold
each about three hundred weight of ore: 120 kibbles are supposed to
clear a cubic fathom of rock.
[Illustration: 712]
_Fig._ 712. represents the wheelbarrow used under ground for conveying
ore and waste to the foot of the shafts. It is made of light deal,
except the wheel, which has a narrow rim of iron.
[Illustration: 713]
_Fig._ 713. represents Mr. Taylor’s ingenious ventilator, or machine for
renewing fresh air in mines. It is so simple in construction, so
complete in its operation, requires so little power to work it, and is
so little liable to injury from wear, that nothing further of the kind
can be desired in ordinary metallic mines. The shaft of the mine is
represented at A; at either the top or bottom of which the machine may
be placed, as is found most convenient, but the foul air must be
discharged into a floor, furnished with a valve-door to prevent its
return into the mine. B is the air-pipe from the mine, passing through
the bottom of the fixed vessel or cylinder C, which is formed of timber,
and bound with iron hoops. It is filled with water nearly to the top of
the pipe B, on which is fixed a valve opening upwards at D. E, the air,
or exhausting cylinder of cast-iron, open at bottom, and suspended over
the air-pipe, but immersed some way in the water. It is furnished with a
wooden top, having an aperture fitted with a valve likewise opening
upwards at F. This exhausting cylinder is moved up and down by the _bob_
G, brought into connexion with any engine by the horizontal rod H; the
weight of the cylinder being balanced, if necessary, by the counterpoise
I. The action is as follows:--When the cylinder rises, the air from the
mine rushes up through the pipe and valve D; and when it descends, this
valve shuts, and prevents the return of the air, which is expelled
through the valve F. With a cylinder two feet in diameter and six feet
long, working from two to three strokes per minute, 200 gallons of air
may be discharged in the same time.
Gunpowder is the most valuable agent of excavation; possessing a power
which has no limit, and which can act every where, even under water. Its
introduction, in 1615, caused a great revolution in the mining art.
It is employed in mines in different manners, and in different
quantities, according to circumstances. In all cases, however, the
process resolves itself into boring a hole, and enclosing a cartridge in
it, which is afterwards made to explode. The hole is always cylindrical,
and is usually made by means of the borer, _fig._ 704., a stem of iron,
terminated by a blunt-edged chisel. It sometimes ends in a cross, formed
by two chisels set transversely. The workman holds the stem in his left
hand, and strikes it with an iron mallet held in his right. He is
careful to turn the punch a very little round at every stroke. Several
punches are employed in succession, to bore one hole; the first shorter,
the latter ones longer, and somewhat thinner. The rubbish is withdrawn
as it accumulates, at the bottom of the hole, by means of a picker,
which is a small spoon or disc of iron fixed at the end of a slender
iron rod. When holes of a large size are to be made, several men must
be employed; one to hold the punch, and one or more to wield the iron
mallet. The perforations are seldom less than an inch in diameter, and
18 inches deep; but they are sometimes 2 inches wide, with a depth of 50
inches.
The gunpowder, when used, is most commonly put up in paper cartridges.
Into the side of the cartridge, a small cylindrical spindle or _piercer_
is pushed. In this state the cartridge is forced down to the bottom of
the hole, which is then stuffed, by means of the tamping bar, _fig._
708., with bits of dry clay, or friable stones coarsely pounded.[33] The
piercer is now withdrawn, which leaves in its place, a channel through
which fire may be conveyed to the charge. This is executed either by
pouring gunpowder into that passage, or by inserting into it, reeds,
straw stems, quills, or tubes of paper filled with gunpowder. This is
exploded by a long match, which the workmen kindle, and then retire to a
place of safety.
[33] Sir Rose Price invented a cap of bronze alloy, to tip the lower
end of the iron rod; a contrivance now generally used in Cornwall.
Before the Geological Society of that county introduced this invention
into practice, scarcely a month elapsed without some dreadful
explosion sending the miner to an untimely grave, or so injuring him
by blowing out his eyes, or shattering his limbs, as to render him a
miserable object of charity for the rest of his days. Scarcely has any
accident happened since the employment of the new tamping-bar. When
the whole bar was made of the tin and copper alloy it was expensive,
and apt to bend; but the iron rod tipped with the bronze is both cheap
and effectual. An ingenious instrument, called the shifting cartridge,
was invented by Mr. Chinalls, and is described in the Transactions of
the above society.
As the _piercer_ must not only be slender, but stiff, so as to be easily
withdrawn when the hole is tamped, iron spindles are usually employed,
though they occasionally give rise to sparks, and consequently to
dangerous accidents, by their friction against the sides of the hole.
Brass piercers have been sometimes tried; but they twist and break too
readily.
Each hole bored in a mine, should be so placed in reference to the
schistose structure of the rock, and to its natural fissures, as to
attack and blow up the least resisting masses. Sometimes the rock is
prepared beforehand for splitting in a certain direction, by means of a
narrow channel excavated with the small hammer.
The quantity of gunpowder should be proportional to the depth of the
hole, and the resistance of the rock; and merely sufficient to split it.
Anything additional would serve no other purpose than to throw the
fragments about the mine, without increasing the useful effect. Into the
holes of about an inch and a quarter diameter, and 18 inches deep, only
two ounces of gunpowder are put.
It appears that the effect of the gunpowder may be augmented by leaving
an empty space above, in the middle of, or beneath the cartridge. In the
mines of Silesia, the consumption of gunpowder has been eventually
reduced, without diminishing the product of the blasts, by mixing
sawdust with it in certain proportions. The hole has also been filled up
with sand in some cases, according to Mr. Jessop’s plan, instead of
being packed with stones, which has removed the danger of the tamping
operation. The experiments made in this way have given results very
advantageous in quarry blasts with great charges of gunpowder; but less
favourable in the small charges employed in mines.
Water does not oppose an insurmountable obstacle to the employment of
gunpowder; but when the hole cannot be made dry, a cartridge bag
impermeable to water must be had recourse to, provided with a tube also
impermeable, in which the _piercer_ is placed.
After the explosion of each mining charge, wedges and levers are
employed, to drag away and break down what has been shattered.
Wherever the rock is tolerably hard, the use of gunpowder is more
economical and more rapid than any tool-work, and is therefore always
preferred. A gallery, for example, a yard and a half high, and a yard
wide, the piercing of which by the hammer formerly cost from five to ten
pounds sterling, the running yard, in Germany, is executed at the
present day by gunpowder at from two to three pounds. When, however, a
precious mass of ore is to be detached, when the rock is cavernous,
which nearly nullifies the action of gunpowder, or when there is reason
to apprehend that the shock caused by the explosion may produce an
injurious fall of rubbish, hand-tools alone must be employed.
In certain rocks and ores of extreme hardness, the use both of tools and
gunpowder becomes very tedious and costly. Examples to this effect are
seen, in the mass of quartz mingled with copper pyrites, worked at
Rammelsberg, in the Hartz, in the masses of stanniferous granite of
Geyer and Altenberg in the Erzgebirge of Saxony, &c. In these
circumstances, fortunately very rare, the action of fire is used with
advantage to diminish the cohesion of the rocks and the ores. The
employment of this agent is not necessarily restricted to these
difficult cases. It was formerly applied very often to the working of
hard substances; but the introduction of gunpowder into the mining art,
and the increase in the price of wood, occasion fire to be little used
as an ordinary means of excavation, except in places where the
scantiness of the population has left a great extent of forest timber,
as happens at Kongsberg in Norway, at Dannemora in Sweden, at Felsobanya
in Transylvania, &c.
The action of fire may be applied to the piercing of a gallery, or to
the advancement of a horizontal cut, or to the crumbling down of a mass
of ore, by the successive upraising of the roof of a gallery already
pierced. In any of these cases, the process consists in forming
bonfires, the flame of which is made to play upon the parts to be
attacked. All the workmen must be removed from the mine during, and even
for some time after, the combustion. When the excavations have become
sufficiently cool to allow them to enter, they break down with levers
and wedges, or even by means of gunpowder, the masses which have been
rent and altered by the fire.
To complete our account of the manner in which man may penetrate into
the interior of the earth, we must point out the form of the excavations
that he should make in it.
In mines, three principal species of excavations may be distinguished;
viz. _shafts_, _galleries_, and the _cavities_ of greater or less
magnitude which remain in the room of the old workings.
A _shaft_ or _pit_ is a prismatic or cylindrical hollow space, the axis
of which is either vertical or much inclined to the horizon. The
dimension of the pit, which is never less than 32 inches in its
narrowest diameter, amounts sometimes to several yards. Its depth may
extend to 1000 feet, and more. Whenever a shaft is opened, means must be
provided to extract the rubbish which continually tends to accumulate at
its bottom, as well as the waters which may percolate down into it; as
also to facilitate the descent and ascent of the workmen. For some time
a wheel and axle erected over the mouth of the opening, which serve to
elevate one or two buckets of proper dimensions, may be sufficient for
most of these purposes. But such a machine becomes ere long inadequate.
Horse-whims, or powerful steam-engines, must then be had recourse to;
and effectual methods of support must be employed to prevent the sides
of the shaft from crumbling and falling down.
A _Gallery_ is a prismatic space, the straight or winding axis of which
does not usually deviate much from the horizontal line. Two principal
species are distinguished; the galleries of _elongation_, which follow
the direction of a bed or a vein; and the _transverse_ galleries, which
intersect this direction under an angle not much different from 90°. The
most ordinary dimensions of galleries are a yard wide, and two yards
high; but many still larger may be seen traversing thick deposits of
ore. There are few whose width is less than 24 inches, and height less
than 40; such small drifts serve merely as temporary expedients in
workings. Some galleries are several leagues in length. We shall
describe in the sequel the means which are for the most part necessary
to support the roof and the walls. The rubbish is removed by waggons or
wheelbarrows of various kinds. See _fig._ 712.
It is impossible to advance the boring of a shaft or gallery beyond a
certain rate, because only a limited set of workmen can be made to bear
upon it. There are some galleries which have taken more than 30 years to
perforate. The only expedient for accelerating the advance of a gallery,
is to commence, at several points of the line to be pursued, portions of
galleries which may be joined together on their completion.
Whether tools or gunpowder be used in making the excavations, they
should be so applied as to render the labour as easy and quick as
possible, by disengaging the mass out of the rock at two or three of its
faces. The effect of gunpowder, wedges, or picks, is then much more
powerful. The greater the excavation, the more important is it to
observe this rule. With this intent, the working is disposed in the form
of _steps_ (_gradins_), placed like those of a stair; each step being
removed in successive portions, the whole of which, except the last, are
disengaged on three sides, at the instant of their being attacked.
The substances to be mined occur in the bosom of the earth, under the
form of alluvial deposits, beds, pipe-veins, or masses, threads or small
veins, and rake-veins.
When the existence of a deposit of ore is merely suspected, without
positive proofs, recourse must be had to labours of research, in order
to ascertain the richness, nature, and disposition of a supposed mine.
These are divided into three kinds; _open workings_, _subterranean
workings_, and _boring operations_.
1. The _working by an open trench_, has for its object to discover the
outcropping or basset edges of strata or veins. It consists in opening a
fosse of greater or less width, which, after removing the vegetable
mould, the alluvial deposits, and the matters disintegrated by the
atmosphere, discloses the native rocks, and enables us to distinguish
the beds which are interposed, as well as the veins that traverse them.
The trench ought always to be opened in a direction perpendicular to the
line of the supposed deposit. This mode of investigation costs little,
but it seldom gives much insight. It is chiefly employed for verifying
the existence of a supposed bed or vein.
The _subterranean workings_ afford much more satisfactory knowledge.
They are executed by different kinds of perforations; viz. by
_longitudinal galleries_ hollowed out of the mass of the beds or veins
themselves, in following their course; by _transverse galleries_, pushed
at right angles to the direction of the veins; by _inclined shafts_,
which pursue the slope of the deposits, and are excavated in their mass;
or, lastly, by _perpendicular pits_.
If a vein or bed unveils itself on the flank of a mountain, it may be
explored, according to the greater or less slope of its inclination,
either by a longitudinal gallery opened in its mass, from the
outcropping surface, or by a transverse gallery falling upon it in a
certain point, from which either an oblong gallery or a sloping shaft
may be opened.
If our object be to reconnoitre a highly inclined stratum, or a vein in
a level country, we shall obtain it with sufficient precision, by means
of shafts, 8 or 10 yards deep, dug at 30 yards distance from one
another; excavated in the mass of ore, in the direction of its deposit.
If the bed is not very much inclined, only 45°, for example, vertical
shafts must be opened in the direction of its roof, or of the
superjacent rocky stratum, and galleries must be driven from the points
in which they meet the ore, in the line of its direction.
When the rocks which cover valuable minerals are not of very great
hardness, as happens generally with the coal formation, with pyritous
and aluminous slates, sal gem, and some other minerals of the secondary
strata, the _borer_ is employed with advantage to ascertain their
nature. This mode of investigation is economical, and gives, in such
cases, a tolerably exact insight into the riches of the interior. The
method of using the borer, has been described under ARTESIAN WELLS.