[Illustration: 835]
A stratum, bed, or seam of coal, is not a solid mass, of uniform
texture, nor always of homogeneous quality in burning. It is often
divided and intersected, with its concomitant strata, by what are named
partings, backs, cutters, reeds, or ends. Besides the chief partings at
the roof and pavement of the coal seam, there are subordinate lines of
parting in the coal mass, parallel to these of variable dimensions.
These divisions are delineated in _fig._ 835., where A, B, C, D, E F G
D, represent a portion of a bed of coal, the parallelogram A B D C the
parting at the roof, and E F G the parting at the pavement; _a b_, _b
c_, _d e_, and _e f_, are the subordinate or intermediate partings; _g
h_, _i k_, _l m_, the backs; _o p_, _p q_, _r s_, _s t_, _u v_, and _v
w_, the cutters. It is thus manifest that a bed of coal, according to
the number of these natural divisions, is subdivided into solid figures
of various dimensions, and of a cubical or rhomboidal shape.
[Illustration: 836 837]
When the engine-pit is sunk, and the lodgement formed, a mine is then
run in the coal to the rise of the field, or a cropping from the
engine-pit to the second pit. This mine may be 6 or 8 feet wide, and
carried either in a line directly to the pit bottom, or at right angles
to the backs or web of the coal, until it is on a line with the pit,
where a mine is set off, upon one side, to the pit bottom. This mine or
gallery is carried as nearly parallel to the backs as possible, till the
pit is gained. _Fig._ 836. represents this mining operation. A is the
engine-pit. B, the second or bye-pit. A C, the gallery driven at right
angles to the backs. C B, the gallery set off to the left hand, parallel
to the backs. The next step is to drive the drip-head or main-levels
from the engine-pit bottom, or from the dip-hand of the backset
immediately contiguous to the engine-pit bottom. In this business, the
best colliers are always employed, as the object is to drive the gallery
in a truly level direction, independently of all sinkings or risings of
the pavement. For coal seams of ordinary thickness, this gallery is
usually not more than 6 feet wide; observing to have on the dip side of
the gallery a small quantity of water, like that of a gutter, so that it
shall always be about 4 or 6 inches deep at the forehead upon the
dip-wall. When the level is driven correctly, with the proper depth of
water, it is said to have dead water at the forehead. In this operation,
therefore, the miner pays no regard to the backs or cutters of the coal;
but is guided in his line of direction entirely by the water-level,
which he must attend to solely, without regard to slips or dislocations
of the strata throwing the coal up or down. In the last figure, the
coal-field is a portion of a basin; so that if the shape be uniform and
unbroken, and if any point be assumed a dipping from the crop, as D, the
level lines from that point will be parallel to the line of crop, as D
E, D F, and the levels from any point whatever a-dipping, will be also
parallel to these; and hence, were the coal-field an entire elliptical
basin, the dip-head levels carried from any point would be elliptical,
and parallel to the crop. If, as is more commonly the case, the
coal-field be merely a portion of a basin, formed by a slip of the
strata, as represented in _fig._ 837., where _a_, _a_, _a_, is the crop,
and A B, a slip of great magnitude, forming another coal-field on the
side C, then the crop not only meets the alluvial cover, but is cut off
by the slip at A and at B. Should any point, therefore, be assigned for
an engine-pit, the levels from it will proceed in a line parallel to the
crop, as D _d_, D _c_, and the level on both sides of the engine-pit
will be also cut off by the slip A B. In this figure, the part included
between the two curve lines, is the breadth or breast of coal-field won
by the engine-pit D; what is not included, is termed the under-dip coal,
and can be worked only by one or more new winnings towards the dip,
according to circumstances.
In British practice, there are four different systems of working
coal-mines:--
1. Working with pillars and rooms, styled post and stall, where the
pillars left, bear such proportion to the coal excavated, as is just
adequate to the support of the incumbent strata.
2. Working with post and stall, where the pillars are left of an extra
size, and stronger than may be requisite for bearing the superior
strata, with the intention of removing a considerable portion of each
massive pillar, whenever the regular working of post and stall has been
finished in the colliery.
3. Working with post and stall, or with comparatively narrow rooms or
boards, whereby an uncommonly large proportion of coal is left, with the
view of working back towards the pits, whenever the colliery is worked
in this manner to the extent of the coal-field, and then taking away
every pillar completely, if possible, and allowing the whole
superincumbent strata to crush down, and follow the miners in their
retreat.
4. Working the long way, being the Shropshire method; which leaves no
pillars, but takes out all the coal progressively as the workings
advance. On this plan, the incumbent strata crush down, creeping very
close to the heads of the miners.
The post and stall system is practised with coals of every thickness.
The Shropshire method is adopted generally with thin coals; for when the
thickness exceeds 6 or 7 feet, this mode has been found impracticable.
The following considerations must be had in view in establishing a
coal-mine:--
1. The lowest coal of the winning should be worked in such a manner as
not to injure the working or the value of the upper coals of the field;
but if this cannot be done, the upper coals should be worked in the
first place.
2. The coals must be examined as to texture, hardness, softness, the
number and openness of the backs and cutters.
3. The nature of the pavement of the coal seam, particularly as to
hardness and softness; and if soft, to what depth it may be so.
4. The nature of the roof of the coal-seam, whether compact, firm, and
strong; or weak and liable to fail; as also the nature of the
superincumbent strata.
5. The nature of the alluvial cover of the ground, as to water,
quicksands, &c.
6. The situation of rivers, lakes, or marshes, particularly if any be
near the outcrop of the coal strata.
7. The situation of towns, villages, and mansion-houses, upon a
coal-field; as to the chance of their being injured by any particular
mode of mining the coal.
Mr. Bald gives the following general rules for determining the best mode
of working coal:--
“1. If the coal, pavement, and roof are of ordinary hardness, the
pillars and rooms may be proportioned to each other, corresponding to
the depth of the superincumbent strata, providing all the coal proposed
to be wrought is taken away by the first working, as in the first
system; but if the pillars are to be winged afterwards, they must be
left of an extra strength, as in the second system.
“2. If the pavement is soft, and the coal and roof strong, pillars of an
extra size must be left, to prevent the pillars sinking into the
pavement, and producing a creep.
“3. If the coal is very soft, or has numerous open backs and cutters,
the pillars must be left of an extra size, otherwise the pressure of the
superincumbent strata will make the pillars fly or break off at the
backs and cutters, the result of which would be a total destruction of
the pillars, termed a crush or sit, in which the roof sinks to the
pavement, and closes up the work.
“4. If the roof is very bad, and of a soft texture, pillars of an extra
size are required, and the rooms or boards comparatively very narrow.
“In short, keeping in view all the circumstances, it may be stated
generally, that when the coal, pavement, and roof are good, any of the
systems before mentioned may be pursued in the working; but if they are
soft, the plan is to work with rooms of a moderate width, and with
pillars of great extra strength, by which the greater part of the coal
may be got out at the last of the work, when the miners retreat to the
pit bottom, and there finish the workings of a pit.”
[Illustration: 838 839]
_Fig._ 838. represents the effects of pillars sinking into the pavement,
and producing a creep; and _fig._ 839. exhibits large pillars and a
room, with the roof stratum bending down before it falls at _a_. Thus
the roads will be shut up, the air-courses destroyed, and the whole
economy of the mining operations deranged.
The proportion of coal worked out, to that left in the pillars, when all
the coal intended to be removed is taken out at the first working,
varies from four-fifths to two-thirds; but as the loss of even
one-third of the whole area of coal is far too much, the better mode of
working suggested in the third system ought to be adopted.
[Illustration: 840]
The proportion of a winning to be worked maybe thus calculated. Let
_fig._ 840. be a small portion of the pillars, rooms, and thirlings
formed in a coal-field; _a_, _a_, are two rooms; _b_, the pillars; _c_,
the thirlings (or area worked out). Suppose the rooms to be 12 feet
wide, the thirlings to be the same, and the pillars 12 feet on each
side; adding the face of the pillar to the width of the room, the sum is
24; and also the end of the pillar to the width of the thirling, the sum
is likewise 24: then 24 × 24 = 576; and the area of the pillar is 12 ×
12 = 144; and as 576 divided by 144, gives 4 for a quotient, the result
is, that one fourth of the coal is left in pillars, and three fourths
extracted. Let _d_, _e_, _f_, _g_, be one winning, and _g_, _e_, _k_,
_h_, another. By inspecting the figure, we perceive the workings of a
coal-field are resolved into quadrangular areas, having a pillar
situated in one of the angles.
[Illustration: 841]
In forming the pillars and carrying forwards the boards with regularity,
especially where the backs and cutters are very distinct and numerous,
it is of importance to work the rooms at right angles to the backs, and
the thirlings in the direction of the cutters, however oblique these may
be to the backs, as the rooms are by this means conducted with the
greatest regularity with regard to each other, kept equidistant, and the
pillars are strongest under a given area. At the same time, however, it
seldom happens that a back or cutter occurs exactly at the place where a
pillar is formed; but this is of no consequence, as the shearing or
cutting made by the miner ought to be in a line parallel to the backs
and cutters. It frequently happens that the dip-head level intersects
the cutters in its progress at a very oblique angle. In this case, when
rooms and pillars are set off, the face of the pillar and width of the
room must be measured off an extra breadth in proportion to the
obliquity, as in _fig._ 841. By neglect of this rule, much confusion and
irregular work is often produced. It is, moreover, proper to make the
first set of pillars next the dip-head level much stronger, even where
there is no obliquity, in order to protect that level from being injured
by any accidental crush of the strata.
[Illustration: 842]
We shall now explain the different systems of working; one of the
simplest of which is shown in _fig._ 842; where A represents the
engine-pit, B the bye-pit, C D the dip-head levels, always carried in
advance of the rooms, and E the rise or crop gallery, also carried in
advance. These galleries not only open out the work for the miners in
the coal-bed, but, being in advance, afford sufficient time for any
requisite operation, should the mines be obstructed by dikes or hitches.
In the example before us, the rooms or boards are worked from the dip to
the crop; the leading rooms, or those most in advance, are on each side
of the crop gallery E; all the other rooms follow in succession, as
shown, in the figure; consequently, as the rooms advance to the crop,
additional rooms are begun at the dip-head level, towards C and D.
Should the coal work better in a level-course direction, then the level
rooms are next the dip-head level, and the other rooms follow in
succession. Hence the rooms are carried a cropping in the one case, till
the coal is cropped out, or is no longer workable; and in the other,
they are extended as far as the extremity of the dip-head level, which
is finally cut off, either by a dike or slip, or by the boundary of the
coal-field.
When the winnings are so very deep as from 100 to 200 fathoms, the first
workings are carried forward with rooms, pillars, and thirlings, but
under a different arrangement, on account of the great depth of the
superincumbent strata, the enormous expense incident to sinking a pit,
and the order and severity of discipline indispensable to the due
ventilation of the mines, the preservation of the workmen, and the
prosperity of the whole establishment. To the celebrated Mr. Buddle the
British nation is under the greatest obligations for devising a new
system of working coal-mines, whereby nearly one-third of the coals has
been rescued from waste and permanent destruction. This system is named
panel work; because, instead of carrying on the coal-field winning in
one extended area of rooms and pillars, it is divided into quadrangular
panels, each panel containing an area of from 8 to 12 acres; and round
each panel is left at first a solid wall of coal from 40 to 50 yards
thick. Through the panel walls roads and air-courses are driven, in
order to work the coal contained within these walls. Thus all the panels
are connected together with the shaft, as to roads and ventilation. Each
district or panel has a particular name; so that any circumstance
relative to the details of the colliery, casualties as to falls and
crushes, ventilation, and the safety of the workmen, can be referred to
a specific place.
[Illustration: 843]
_Fig._ 843. represents a part of a colliery laid out in four panels,
according to the improved method. To render it as distinct as possible,
the line of the boards is at right angles with the dip-head level, or
level course of the coal. A is the engine-shaft, divided into three
compartments, an engine-pit and two coal-pits, like _fig._ 825. One of
the coal-pits is the down-cast, by which the atmospheric air is drawn
down to ventilate the works; the other coal-pit is the up-cast shaft, at
whose bottom the furnace for rarefying the air is placed. B C, is the
dip-head level; A E, the rise or crop gallery; K, K, the panel walls; F,
G, are two panels completed as to the first work; D, is a panel, with
the rooms _a_, _a_, _a_, in regular progress to the rise; H, is a panel
fully worked out, whence nearly all the coal has been extracted; the
loss amounting in general to no more than a tenth, instead of a third,
or even a half, by the old method. By this plan of Mr. Buddle’s, also,
the pillars of a panel may be worked out at any time most suitable for
the economy of the mining operation; whereas formerly, though the size
of the pillars and general arrangement of the mine were made with the
view of taking out ultimately a great proportion of the pillars, yet it
frequently happened that, before the workings were pushed to the
proposed extent, some part of the mine gave way, and produced a crush;
but the most common misfortune was the pillars sinking into the
pavement, and deranging the whole economy of the field. Indeed the crush
or creep often overran the whole of the pillars, and was resisted only
by the entire body of coal at the wall faces; so that the ventilation
was entirely destroyed, the roads leading from the wall faces to the
pit-bottom shut up and rendered useless, and the recovery of the
colliery by means of new air-courses, new roads, and by opening up the
wall faces or rooms, was attended with prodigious expense and danger.
Even when the pillars stood well, the old method was attended with other
very great inconveniences. If water broke out in any particular spot of
the colliery, it was quite impossible to arrest its progress to the
engine-pit; and if the ventilation was thereby obstructed, no idea could
be formed where the cause might be found, there being instances of no
less than 30 miles of air-courses in one colliery. And if from
obstructed ventilation an explosion of the fire-damp occurred while many
workmen were occupied along the extended wall faces, it was not possible
to determine where the disaster had taken place; nor could the viewers
and managers know where to bring relief to the forlorn and mutilated
survivors.
In Mr. Buddle’s system all these evils are guarded against, as far as
human science and foresight can go. He makes the pillars very large, and
the rooms or boards narrow; the pillars being in general 12 yards broad,
and 24 yards long; the boards 4 yards wide, and the walls or thirlings
cut through the pillars from one board to another, only 5 feet wide, for
the purpose of ventilation. In the figure, the rooms are represented as
proceeding from the dip to the crop, and the panel walls act as barriers
thrown round the area of the panel, to prevent the weight of the
superincumbent strata from overrunning the adjoining panels. Again, when
the _pillars_ of a panel are to be worked, one range of pillars, as at I
(in H), is first attacked; and as the workmen cut away the furthest
pillars, columns of prop-wood are erected betwixt the pavement and the
roof, within a few feet of each other (as shown by the dots), till an
area of above 100 square yards is cleared of pillars, presenting a body
of-strata perhaps 130 fathoms thick, suspended clear and without
support, except at the line of the surrounding pillars. This operation
is termed working the _goaff_. The only use of the prop-wood is to
prevent the seam, which forms the ceiling over the workmen’s heads, from
falling down and killing them by its splintery fragments. Experience has
proved, that before proceeding to take away another set of pillars, it
is necessary to allow the last-made goaff to fall. The workmen then
begin to draw out the props, which is a most hazardous employment. They
begin at the more remote props, and knock them down one after another,
retreating quickly under the protection of the remaining props.
Meanwhile the roof-stratum begins to break by the sides of the pillars,
and falls down in immense pieces; while the workmen still persevere,
boldly drawing and retreating till every prop is removed. Nay, should
any props be so firmly fixed by the top pressure, that they will not
give way to the blows of heavy mauls, they are cut through with axes;
the workmen making a point of honour to leave not a single prop in the
goaff. The miners next proceed to cut away the pillars nearest to the
sides of the goaff, setting prop-wood, then drawing it, and retiring as
before, until every panel is removed, excepting small portions of
pillars which require to be left under dangerous stones to protect the
retreat of the workmen. While this operation is going forward, and the
goaff extending, the superincumbent strata being exposed without support
over a large area, break progressively higher up; and when strong beds
of sandstone are thus giving way, the noise of the rending rocks is very
peculiar and terrific; at one time loud and sharp, at another hollow and
deep.
As the pillars of the panels are taken away, the panel walls are also
worked progressively backwards to the pit bottom; so that only a very
small proportion of coal is eventually lost. This method is undoubtedly
the best for working such coals as those of Newcastle, considering their
great depth beneath the surface, their comparative softness, and the
profusion of inflammable air. It is evident that the larger the pillars
and panel walls are, in the first working, the greater will be the
security of the miners, and the greater the certainty of taking out, in
the second stage, the largest proportion of coal. This system may be
applied to many of the British collieries; and it will produce a vast
quantity of coals beyond the post and stall methods, so generally
persisted in.
In thus tearing to pieces the massive rocks over his head, the miner
displays a determined and cool intrepidity; but his ingenuity is no less
to be admired in contriving modes of carrying currents of pure
atmospheric air through every turning of his gloomy labyrinth, so as to
sweep away the explosive spirit of the mine.
The fourth system of working coal, is called the _long way_, the
long-wall, and the Shropshire method. The plan must at first have been
extremely hazardous; though now it is so improved as to be reckoned as
safe, if not safer, to the workmen, than the other methods, with rooms
and pillars.
The object of the Shropshire system, is to begin at the pit-bottom
pillars, and to cut away at once every inch of coal progressively
forward, and to allow the whole superincumbent strata to crush down
behind and over the heads of the workmen. This plan is pursued chiefly
with coals that are thin, and is very seldom adopted when the seam is 7
feet thick; from 4 to 5 feet being reckoned the most favourable
thickness for proceeding with comfort, amidst ordinary circumstances, as
to roof, pavement, &c. When a pit is opened on a coal to be treated by
this method, the position of the coals above the lowest seam sunk to,
must first be considered; if the coal beds be contiguous, it will be
proper to work the upper one first, and the rest in succession
downwards; but if they are 8 fathoms or more apart, with strata of
strong texture betwixt them, the working of the lower coals in the first
place will do no injury to that of the upper coals, except breaking
them, perhaps, a little. In many instances, indeed, by this operation on
a lower coal, upper coals are rendered more easily worked.
[Illustration: 844]
When the operation is commenced by working on the Shropshire plan, the
dip-head levels are driven in the usual manner, and very large bottom
pillars are formed, as represented in _fig._ 844. Along the rise side of
the dip-head level, chains of wall, or long pillars, are also made, from
8 to 10 yards in breadth, and only mined through occasionally, for the
sake of ventilation, or of forming new roads. In other cases no pillars
are left upon the rise side of the level; but, instead of them,
buildings of stone are reared, 4 feet broad at the base, and 9 or 10
feet from the dip side of the level. Though the roads are made 9 feet
wide at first, they are reduced to half that width after the full
pressure of the strata is upon them. Whenever these points are secured,
the operation of cutting away the whole body of the coal begins. The
place where the coal is removed, is named the _gobb_ or waste; and
gobbin, or gobb-stuff, is stones or rubbish taken away from the coal,
pavement, or roof, to fill up that excavation as much as possible, in
order to prevent the crush of superincumbent strata from causing heavy
falls, or following the workmen too fast in their descent. Coals mined
in this manner work most easily according to the way in which the widest
backs and cutters are; and therefore, in the Shropshire mode, the walls
stand sometimes in one direction, and sometimes in another; the mine
always turning out the best coals when the open backs and cutters face
the workmen. As roads must be maintained through the crushed strata, the
miners in the first place cut away about 15 feet of coal round the
pit-bottom pillars, and along the upper sides of the dip-head chain
walls; and then, at the distance of 9 or 10 feet, carry regular
buildings of stone 3 feet broad, with props set flush with the faces of
these, if necessary. As the miners advance, they erect small pillars of
roof or pavement stone in regular lines with the wall face, and
sometimes with props intermediate.
There are two principal modifications of the Shropshire plan. The first,
or the original system, was to open out the wall round the pit-bottom;
and, as the wall face extended, to set off main roads and branches, very
like the branches of a tree. These roads were so distributed, that
between the ends of any two branches there should be a distance of 30 or
40 yards, as might be most convenient. (see _fig._ 844.) Each space of
coal betwixt the roads is called a wall; and one half of the coals
produced from each wall is carried to the one road, and the other half
to the other road. This is a great convenience when the roof is bad; and
hence a distance of only 20 yards betwixt the roads is in many instances
preferred. In _fig._ 844. A represents the shaft; B B, the wall-face;
_a_, the dip-head level; _b_, the roads, from 20 to 40 yards asunder;
_c_, the _gobb_ or waste, with buildings along the sides of the roads;
and _d_, the pillars.
[Illustration: 845]
The other Shropshire system is represented in _fig._ 845., where A shows
the pit, with the bottom pillars; _b_, the dip-head levels; _c_, the
off-break from the level, where no pillars are left; _d_, the off-break,
where pillars remain to secure the level. All roads are protected in the
sides by stone buildings, if they can be had, laid off 9 feet wide.
After the crush settles, the roads generally remain permanently good,
and can, in many cases, be travelled through as easily 50 years after
they have been made, as at the first. Should stones not be forthcoming,
coals must be substituted, which are built about 20 inches in the base.
In this method, the roads are likewise from 20 to 40 yards apart; but
instead of ramifying, they are arranged parallel to each other. The
miners secure the waste by gobbing; and three rows of props are carried
forwards next the wall faces _a_, with pillars of stone or of coal
reared betwixt them. This mode has a more regular appearance than the
other; though it is not so generally practised.
In the post and stall system, each man has his own room, and performs
all the labour of it; but in that of Shropshire, there is a division of
labour among the workmen, who are generally divided into three
companies. The first set curves or pools the coal along the whole line
of walls, laying in or pooling at least 3 feet, and frequently 45
inches, or 5 quarters, as it is called. These men are named _holers_. As
the crush is constantly following them, and impending over their heads,
causing frequent falls of coal, they plant props of wood for their
protection at regular distances in an oblique direction between the
pavement and wall face. Indeed, as a further precaution, staples of
coal, about 10 inches square, are left at every 6 or 8 yards, till the
line of holing or curving is completed. The walls are then marked off
into spaces of from 6 to 8 yards in length; and at each space a shearing
or vertical cut is made, as deep as the holing; and when this is done,
the holer’s work is finished. The set who succeed the holers, are called
getters. These commence their operations at the centre of the wall
divisions, and drive out the _gibbs_ and staples. They next set wedges
along the roof, and bring down progressively each division of coal; or,
if the roof be hard-bound, the coal is blown down with gunpowder. When
the roof has a good parting, the coals frequently fall down the moment
the gibbs are struck; which makes the work very easy. The getters are
relieved in their turn by the third set, named butty-men, who break down
the coals into pieces of a proper size for sending up the shaft, and
take charge of turning out the coal from the wall face to the ends of
the roads. This being done, they build up the stone pillars, fill up the
gobb, set the trees, clear the wall faces of all obstructions, set the
gibbs, and make every thing clear and open for the holers to resume
their work. If the roads are to be heightened by taking down the roof,
or removing the pavement, these butty-men do this work also, building
forwards the sides of the roads, and securing them with the requisite
props. When a coal has a following or roof stone, which regularly
separates with the coal, this facilitates the labour, and saves much of
the coal; and should a soft bed of fire-clay occur a foot or two
beneath the coal-seam, the holing is made in it, instead of into the
coal, and the stone betwixt the holing and the coal benched down, which
serves for pillars and gobbing. In this way all the vendible coal
becomes available.
[Illustration: 846]
Another form of the Shropshire system is, for each miner to have from 6
to 12 feet of coal before him, with a leading-hand man; and for the
several workmen to follow in succession, like the steps of a stair. When
the coal has open backs and cutters, this work goes on very regularly,
as represented in _fig._ 846., where the leading miner is at _a_, next
to the outcrop, and _b b_, &c. are the wall faces of each workman; A
being the shaft, and B the dip-head level. In this case the roads are
carried either progressively through the gobb, or the gobb is entirely
shut up; and the whole of the coals are brought down the wall-faces,
either to the dip-head level or the road _c_, _c_. This method may be
varied by making the walls broad enough to hold two, three, or four men
when each set of miners performs the whole work of holing, getting,
breaking down, and carrying off the coals.
It is estimated that from one-eighth to one-twelfth part only of the
coals remains underground by the Shropshire plan; nay, in favourable
circumstances, almost every inch of coal may be taken out, as its
principle is to leave no solid pillars nor any coal below, except what
may be indispensable for securing the gobb. Indeed this system might be
applied to coal seams of almost any ordinary thickness, providing stuff
to fill up the gobb could be conveniently procured.
In Great Britain, seams of coal are mined when they are only 18 inches
thick; but if thinner, the working of fire-clay or ironstone immediately
adjoining must be included. A few instances may be adduced, indeed,
where caking coals of a fine quality for blacksmiths have been worked,
though only in 12-inch seams.
Eighteen-inch seams are best worked by young lads and boys. The coal
itself may be mined without lifting the pavement, or taking down the
roof in the rooms; but roads must be cut either in the pavement or the
roof, for removing the coals to the pit-bottom. All coals less than 2
feet 3 inches thick, are worked with the view of taking out all the
coal, either on the Shropshire system, or with pillar-walls and rooms;
with this peculiarity, that, on account of the thinness of the seam, the
rooms are worked as wide as the roof will bear up; or if a following of
the roof-stone, or fall of it, can be brought on, it proves
advantageous, by not only giving head-room, but by filling up the waste,
and rendering the roads easily kept for the working of the pillars.
Where no following takes place, small temporary pillars, about 8 feet
square, are left along the chain-wall side. The walls may vary in
thickness from 4 to 16 yards, according to circumstances, and they are
holed through only for ventilation.
Coals from 5 to 8 feet thick are the best suited in every point of view
for the effective work of the miner, and for the general economy of
underground operations. When they exceed that thickness, they require
very excellent roofs and pavements, to render the working either safe or
comfortable; or to enable those who superintend the field to get out a
fair proportion of coal from a given area. In such powerful beds the
Shropshire method is impracticable, from want of gobbin; and long props,
unless of prodigious girth, would present an inadequate resistance to
the pressure of the massive ceiling.
[Illustration: 847]
When coals do not exceed 20 feet in thickness, and have good roofs, they
are sometimes worked as one bed of coal; but if the coal be tender or
free, it is worked as two beds. One-half of such thick coal, however, is
in general lost in pillars; and it is very seldom that less than
one-third can be left. When the coal is free and ready to crumble by the
incumbent pressure, as well as by the action of the air, the upper
portion of the coal is first worked, then a scaffolding of coal is left,
2 or 3 feet thick, according to the compactness of the coal; and the
lower part of the coal is now worked, as shown in _fig._ 847. As soon as
the workings are completed to the proposed extent, the coal scaffoldings
are worked away, and as much of the pillars as can be removed with
safety. As propwood is of no use in coal seams of such a height, and as
falls from the roof would prove frequently fatal to the miners, it is
customary with tender roofs to leave a ceiling of coal from 2 to 3 feet
thick. This makes an excellent roof; and should it break, gives warning
beforehand, by a peculiar crackling noise, very different from that of
roof-stones crushing down.
One of the thickest coals in Great Britain, worked as one bed from roof
to pavement, is the very remarkable seam near the town of Dudley, known
by the name of the ten-yard coal, about 7 miles long, and 4 broad. No
similar coal has been found in the island; and the mode of working it is
quite peculiar, being a species of panel work totally different from
the modern Newcastle system. A compartment, or pannel, formed in working
the coal, is called a side of work; and as the whole operation is
exhibited in one of these compartments, it will be proper to describe
the mode of taking the coal from one of them, before describing the
whole extent of the workings of a mine.
[Illustration: 848]
Let _fig._ 848. represent a side of work; A, the ribs or walls of coal
left standing round, constituting the side of work; _a_, the pillars, 8
yards square; _c_, the stalls, 11 yards wide; _d_, the cross openings,
or through puts, also 11 yards wide; _e_, the bolt-hole, cut through the
rib from the main road, by which bolt-hole the side of work is opened
up, and all the coals removed. Two, three, or even four bolt-holes open
into a side of work, according to its extent; they are about 8 feet
wide, and 9 feet high. The working is in a great measure regulated by
the natural fissures and joints of the coal-seam; and though it is 30
feet thick, the lower band, of 2 feet 3 inches, is worked first; the
miners choosing to confine themselves within this narrow opening, in
order to gain the greater advantage afterwards, in working the
superjacent coal. Whenever the bolt hole is cut through, the work is
opened up by driving a gallery forward, 4 feet wide, as shown by the
dotted lines. At the sides of this gallery next the bolt-hole, each
miner breaks off in succession a breast of coal, two yards broad, as at
_f_, _f_, by means of which the sides of the rib-walls A, are formed,
and the area of the pillars. In this way each collier follows another,
as in one of the systems of the Shropshire plan. When the side of work
is laid open along the rib-walls, and the faces and sides of the pillars
have been formed, the upper coals are then begun to be worked, next the
rib-wall. This is done by shearing up to a bed next the bolt-hole, and
on each side, whereby the head coals are brought regularly down in large
cubical masses, of such thickness as suits with the free partings or
subordinate divisions of the coals and bands. Props of wood, or even
stone pillars, are placed at convenient distances for the security of
the miners.
In working the ten-yard coal, a very large proportion of it is left
underground, not merely in pillars and rib-walls, but in the state of
small coal produced in breaking out the coal. Hence, from four-tenths to
a half of the total amount is lost for ever.
[Illustration: 849]
Another method of working coal of uncommon thickness, is by scaffoldings
or stages of coals, as practised in the great coal bed at Johnstone,
near Paisley, of which a section has already been given. In one part of
the field the coal is from 50 to 60 feet thick, and in another it
amounts to 90 feet. The seams of stone interspersed through the coal are
generally inconsiderable, and amount in only two cases to 27 inches in
thickness. The roof of the coal is so unsound, and the height so
prodigious, that it could not possibly be worked in one seam, like that
of Staffordshire. About 3 feet of the upper coal is therefore left as a
roof, under which a band of coal, from 6 to 7 feet thick, is worked on
the post and stall plan, with square pillars of extra strength, which
are thereafter penetrated. A platform about 3 feet high is left at the
sole; under which the rooms and pillars are set off and worked in
another portion of the coal, from 5 to 7 feet thick, great care being
had to place pillar under pillar, and partition under partition, to
prevent a crush. Where the coal is thickest, no less than 10 bands of it
are worked in this way, as is shown in _fig._ 849. When any band of the
coal is foul from sulphur or other causes, it is left for the next
platform, so that a large proportion of it is lost, as in the
Staffordshire mines. Much attention must here be paid to the vertical
distribution of the pillars and apartments; the miner’s compass must be
continually consulted, and bore-holes must be put down through the coal
scaffoldings, to regulate correctly the position of the pillars under
one another.
_Edge coals_, which are nearly perpendicular, are worked in a peculiar
manner; for the collier stands upon the coal, having the roof on the one
hand, and the floor on the other, like two vertical walls. The
engine-pit is sunk in the most powerful stratum. In some instances the
same stratum is so vertical as to be sunk through for the whole depth of
the shaft.
[Illustration: 850]
Whenever the shaft has descended to the required depth, galleries are
driven across the strata from its bottom, till the coals are
intersected, as is shown in _fig._ 850., where we see the edge coals at
_a_, _a_; A, the engine-pit; _b_, _b_, the transverse galleries from the
bottom of the shaft; and _c_, _c_, upper transverse galleries, for the
greater conveniency of working the coal. The principal edge coal works
in Great Britain lie in the neighbourhood of Edinburgh, and the coals
are carried on the backs of women from the wall-face to the bottom of
the engine-pit.
The modes of carrying coals from the point where they are excavated to
the pit bottom, are nearly as diversified as the systems of working.
One method employs hutches, or baskets, having slips or cradle feet shod
with iron, containing from 2 to 3 hundred weight of coals. These baskets
are dragged along the floor by ropes or leather harness attached to the
shoulders of the workmen, who are either the colliers or persons hired
on purpose. This method is used in several small collieries; but it is
extremely injudicious, exercising the muscular action of a man in the
most unprofitable manner. Instead of men, horses are sometimes yoked to
these basket-hurdles, which are then made to contain from 4 to 6 hundred
weight of coals; but from the magnitude of the friction, this plan
cannot be commended.
An improvement on this system, where men draw the coals, is to place the
basket or corve on a small four-wheeled carriage, called a tram, or to
attach wheels to the corve itself. Thus much more work is performed,
provided the floor be hard; but not on a soft pavement, unless some kind
of wooden railway be laid.
[Illustration: 851]
The transport of coals from the wall-face to the bottom of the shaft,
was greatly facilitated by the introduction of cast-iron railways, in
place of wooden roads, first brought into practice by Mr. John Curr of
Sheffield. The rails are called tram-rails, or plate-rails, consisting
of a plate from 3 to 4 inches broad, with an edge at right angles to it
about two inches and a half high. Each rail is from 3 to 4 feet long,
and is fixed either to cross bearers of iron, called sleepers, or more
usually to wooden bearers. In some collieries, the miners, after working
out the coals, drag them along these railways to the pit bottom; but in
others, two persons called trammers are employed to transport the coals;
the one of whom, in front of the corve, draws with harness; and the
other, called the patter, pushes behind. The instant each corve arrives,
from the wall-face, at a central spot in the system of the railways, it
is lifted from the tram by a crane placed there, and placed on a
carriage called a rolley, which generally holds two corves. Whenever
three or four rolleys are loaded, they are hooked together, and the
rolley driver, with his horse, takes them to the bottom of the
engine-shaft. The rolley horses have a peculiar kind of shafts, commonly
made of iron, named limbers, the purpose of which is to prevent the
carriage from overrunning them. One of these shafts is represented in
_fig._ 851. The hole shown at _a_, passes over an iron peg or stud in
front of the rolley, so that the horse may be quickly attached or
disengaged. By these arrangements the work is carried on with surprising
regularity and despatch.
The power of the engine for drawing the coals up the shaft, is made
proportional to the depth of the pit and the quantity to be raised, the
corves ascending at an average velocity of about 12 feet per second. So
admirable is the modern arrangement of this operation, that the corves
are transported from the wall-faces to the pit bottom, and moved up the
shaft, as fast as the onsetters at the bottom, and the banksmen at the
top, can hook the loaded and empty corves on and off the engine ropes.
Thus 100 corves of coals have been raised every hour up a shaft 100
fathoms deep, constituting a lift of 27 tons per hour, or 324 tons in a
day, or shift of 12 hours. Coals mined in large cubical masses cannot,
however, be so rapidly raised as the smaller coal of the Newcastle
district.
When coals have so great a rise from the pit bottom to the crop that
horses cannot be used on the rolley ways, the corves descend along the
tram-roads, by means of inclined-plane machines, which are moved either
by vertical rope-barrels, or horizontal rope-sheaves. These inclined
planes are frequently divided into successive stages, 200 or 300 yards
long, at the end of each of which is an inclined-plane machine, whereby
the coals are lowered from one level to another.
The wheels of the trams and rolleys vary in diameter from 8 to 16
inches, according to the thickness of the coal. In some, the axles not
only revolve on their journals, but the wheels also revolve on their
axles.
Various forms of machines have been employed for raising the coals out
of the pits. The steam engine with fly-wheel and rope-barrels, is,
however, now preferred in all considerable establishments. When of small
power, they are usually constructed with a fly wheel, and short
fly-wheel shaft, on which there is a small pinion working into the teeth
of a large wheel, fixed upon the rope barrel. Thus the engine may move
with great rapidity, while it imparts an equable slow motion to the
corves ascending in the shaft. When the engines are of great power,
however, they are directly connected with the rope-barrel; some of these
being of such dimensions, that each revolution of the rope-barrel
produces an elevation of 12 yards in the corve. A powerful brake is
usually connected with the circumference of the fly-wheel or
rope-barrel, whereby the brakeman, by applying his foot to the governing
lever of the brake, and by shutting at the same time the steam valves
with his hands, can arrest the corve, or pitch its arrival within a few
inches of the required height of every delivery. An endless chain,
suspended from the bottom to the top of the shaft, has, in a few pits of
moderate depth, been worked by a steam engine, for raising corves in
constant succession; but the practice has not been found hitherto
applicable on the greater scale.
There is a kind of water engines for raising coals, strictly admissible
only in level free pits, where the ascent of the loaded corve is
produced by the descent of a cassoon filled with water. When the ascent
and descent are through equal spaces, the rope barrels for the cassoon
and the corves are of equal diameter; but when the point from which the
coals have to be lifted is deeper than the point of discharge for the
water into the dry level, the cassoon must be larger, and the rope
barrel smaller; so that by the time the cassoon reaches to the
half-depth, for example, the corve may have mounted through double the
space. The cassoon is filled with water at the pit mouth, and is emptied
by a self-acting valve whenever it gets to the bottom. The loaded corve
is replaced by an empty one at the pit mouth, and its weight, with that
of the descending rope, pull up the empty cassoon; the motions of the
whole mechanism being regulated by a powerful brake.
Various plans have been devised to prevent collision between the
ascending and descending corves, which sometimes pass each other with a
joint velocity of 20 or 30 feet per second. One method is by dividing
the pit from top to bottom, so that each corve moves in a separate
compartment. Another mode was invented by Mr. Curr of Sheffield, in
which wooden guides were attached from top to bottom of the pit; being
spars of deal about 4 inches square, attached perpendicularly to the
sides of the shaft, and to buntons in the middle of the pit. Betwixt
these guides, friction-roller sliders are placed, attached to the
gin-ropes, to which sliders the corves are suspended. In this way, the
corves can be raised with great rapidity; but there is a considerable
loss of time in banking the corve at the pit mouth, where shutters or
sliding boards must be used. This plan is highly beneficial where the
coals are in large lumps.
Both ropes and chains are used for lifting coals. The round ropes are
shroud-laid; but the preferable rope is the flat band, made of four
ropes placed horizontally together, the ropes being laid alternately
right and left. In this way, the ropes counteract one another in the
twist, hanging like a ribbon down the shaft; and are stitched strongly
together by a small cord. Such rope bands are not only very pliable for
their strength, which protects the heart of the rope from breaking, but
as they lap upon themselves, a simple sheave serves as a rope-barrel.
They possess the additional advantage, that by so lapping, they enlarge
the diameter of the axle in which they coil, and thus make a
compensation mechanically against the increasing length of rope
descending with its corve. Thus the counterpoise chains, used in deep
pits to regulate the descent, have been superseded. See ROPE-SPINNING.
When chains are preferred to ropes, as in very deep pits, the short
pudding-link chains are mostly used. See CABLE.
The corves after being landed or banked at the pit mouth, are drawn to
the bin or coal-hill, either upon slips by horses, or by trammers on a
tram-road. But with small coals, like the Newcastle, the pit head is
raised 8 or 9 feet above the common level of the ground, and the
coal-heap slopes downwards from that height. As the bins increase,
tram-roads are laid outwards upon them.
I shall now describe the _ventilation_ of coal mines. Into their
furthest recesses, an adequate supply of fresh air must be carried
forwards, for the purposes of respiration, and the combustion of
candles; as also for clearing off the carbonic acid and carburetted
hydrogen gases, so destructive to the miners, who call these noxious
airs, from their most obvious qualities, choke-damp and fire-damp.
Before the steam engine was applied to the drainage of the mines, and
the extraction of the coal, the excavations were of such limited extent,
that when inflammable air accumulated in the foreheads, it was usual in
many collieries to fire it every morning. This was done by fixing a
lighted candle to the end of a long pole, which being extended towards
the roof by a person lying flat on the floor, the gas was fired, and the
blast passed safely over him. If the gas was abundant, the explosive
miner put on a wet jacket, to prevent the fire from scorching him. In
other situations, where the fire-damp was still more copious, the candle
was drawn forwards into it, by a cord passing over a catch at the end of
the gallery, while the operator stood at a distance. This very rude and
dangerous mode of exploding the inflammable gas, is still practised in a
few mines, under the name of the firing line.
The carbonic acid or choke-damp having a greater specific gravity than
atmospheric air, in the proportion of about 3 to 2, occupies the lower
part of the workings, and gives comparatively little annoyance. Its
presence may moreover be always safely ascertained by the lighted
candle. This cannot, however, be said of the fire-damp, which being
lighter and more movable, diffuses readily through the atmospheric air,
so as to form a most dangerous explosive mixture, even at a considerable
distance from the blowers or sources of its extrication from the coal
strata. Pure subcarburetted hydrogen has a specific gravity = 0·555, air
being 1; and consists of a volume of vapour of carbon, and two volumes
of hydrogen, condensed by mutual affinity into one volume. The
choke-damp is a mixture of the above, with a little carbonic acid gas,
and variable proportions of atmospheric air. As the pure subcarburetted
hydrogen requires twice its bulk of oxygen to consume it completely, it
will take for the same effect about 10 times its bulk of atmospheric
air, since this volume of air contains about two volumes of oxygen. Ten
volumes of air, therefore, mixed with one volume of subcarburetted
hydrogen, form the most powerfully explosive mixture. If either less or
more air be intermixed, the explosive force will be impaired; till 3
volumes of air below or above that ratio, constitute non-explosive
mixtures; that is, 1 of the pure fire-damp mixed with either 7 or 13 of
air, or any quantity below the first, or above the second number, will
afford an unexplosive mixture. With the first proportion, a candle will
not burn; with the second, it burns with a very elongated blue flame.
The fire-damp should therefore be still further diluted with common air,
considerably beyond the above proportion of 1 to 13, to render the
working of the mine perfectly safe.
These noxious gases are disengaged from the cutters, fissures, and
minute pores of the coal; and if the quantity be considerable, relative
to the orifice, a hissing noise is heard.
[Illustration: 852]
Though the choke-damp, or carbonic acid gas, be invisible, yet its line
of division from the common air is distinctly observable on approaching
a lighted candle to the lower level, where it accumulates, which becomes
extinguished the instant it comes within its sphere, as if it were
plunged in water. The stratum of carbonic acid sometimes lies 1 or 2
feet thick on the floor, while the superincumbent air is perfectly good.
When the coal has a considerable dip and rise, the choke-damp will be
found occupying the lower parts of the mine, in a wedge form, as
represented in _fig._ 852., where _a_ shows the place of the carbonic
acid gas, and _b_ that of the common air.
When a gallery is driven in advance of the other workings, and a
discharge of this gas takes place, it soon fills the whole mine, if its
direction be in the line of level, and the mine is rendered unworkable
until a supply of fresh air is introduced to dislodge it. As the flame
of a candle indicates correctly the existence of the choke-damp, the
miners may have sufficient warning of its presence, so as to avoid the
place which it occupies, till adequate means be taken to drive it away.
The fire-damp is not an inmate of every mine, and is seldom found,
indeed, where the carbonic acid prevails. It occurs in the greatest
quantities in the coal mines of the counties of Northumberland, Durham,
Cumberland, Staffordshire, and Shropshire. It is more abundant in coals
of the caking kind, with a bright steel-grained fracture, than in cubic
coals of an open-burning quality. Splint coals are still less liable to
disengage this gas. In some extensive coal-fields it exists copiously on
one range of the line of bearing, while on the other range, none of it
is observed, but abundance of carbonic acid gas.
In the numerous collieries in the Lothians, south from the city of
Edinburgh, the fire-damp is unknown; while in the coal-fields round the
city of Glasgow, and along the coast of Ayrshire, it frequently appears.
[Illustration: 853]
The violent discharge of the gas from a crevice or cutter of the coal,
is called a blower; and if this be ignited, it burns like an immense
blowpipe, inflaming the coal at the opposite side of the gallery. The
gas evidently exists in a highly compressed and elastic state; whence it
seems to loosen the texture of the coals replete with it, and renders
them more easily worked. The gas is often peculiarly abundant near a
great dislocation or slip of the strata; so that the fissure of the
dislocation will sometimes emit a copious stream of gas for many years.
It has also happened, that from certain coals, newly worked, and let
fall from a height into the hold of a vessel, so much inflammable gas
has been extricated that, after the hatches were secured, and the ship
ready to proceed to sea, the gas has ignited with the flame of a candle,
so as to scorch the seamen, to blow up the decks, and otherwise damage
the vessel. In like manner, when the pillars in a mine are crushed by
sudden pressure, a great discharge of gas ensues. This gas being lighter
than common air, always ascends to the roof or to the rise of the
galleries; and, where the dip is considerable, occupies the forehead of
the mine, in a wedge form, as shown in _fig._ 853., where _a_ represents
the fire-damp, and _b_ the common air. In this case, a candle will burn
without danger near the point _c_, close to the floor; but if it be
advanced a few feet further towards the roof, an explosion will
immediately ensue; since at the line where the two elastic fluids are in
contact, they mix, and form an explosive body.
[Illustration: 854]
When this gas is largely diluted with air, the workmen do not seem to
feel any inconvenience from breathing the mixture for a period of many
years; but on inhaling pure carburetted hydrogen, the miner instantly
drops down insensible, and, if not speedily removed into fresh air, he
dies. The production of these noxious gases renders ventilation a
primary object in the system of mining. The most easily managed is the
carbonic acid. If an air-pipe has been carried down the engine pit for
the purpose of ventilation in the sinking, other pipes are connected
with it, and laid along the pavement, or are attached to an angle of the
mine next the roof. These pipes are prolonged with the galleries, by
which means the air at the forehead is drawn up the pipes and replaced
by atmospheric air, which descends by the shaft in an equable current,
regulated by the draught of the furnace at the pit mouth. This
circulation is continued till the miners cut through upon the second
shaft, when the air-pipes become superfluous; for it is well known that
the instant such communication is made, as is represented in _fig._
854., the air spontaneously descends in the engine pit A, and, passing
along the gallery _a_, ascends in a steady current in the second pit B.
The air, in sinking through A, has at first the atmospheric temperature,
which in winter may be at or under the freezing point of water; but its
temperature increases in passing down through the relatively warmer
earth, and ascends in the shaft B, warmer than the atmosphere. When
shafts are of unequal depths, as represented in the figure, the current
of air flows pretty uniformly in one direction. If the second shaft has
the same depth with the first, and the bottom and mouth of both be in
the same horizontal plane, the air would sometimes remain at rest, as
water would do in an inverted syphon, and at other times would circulate
down one pit and up another, not always in the same direction, but
sometimes up the one, and sometimes up the other, according to the
variations of temperature at the surface, and the barometrical
pressures, as modified by winds. There is in mines a proper heat,
proportional to their depth, increasing about one degree of Fahrenheit’s
scale for every 60 feet of descent.
[Illustration: 855]
There is a simple mode of conducting air from the pit bottom to the
forehead of the mine, by cutting a ragglin, or trumpeting, as it is
termed, in the side of the gallery, as represented in _fig._ 855., where
A exhibits the gallery in the coal, and B the ragglin, which is from 15
to 18 inches square. The coal itself forms three sides of the air-pipe,
and the fourth is composed of thin deals applied air-tight, and nailed
to small props of wood fixed between the top and bottom of the lips of
the ragglin. This mode is very generally adopted in running galleries of
communication, and dip-head level galleries, where carbonic acid
abounds, or when from the stagnation of the air the miners’ lights burn
dimly.
When the ragglin or air-pipes are not made spontaneously active, the air
is sometimes impelled through them by means of ventilating fanners,
having their tube placed at the pit bottom, while the vanes are driven
with great velocity by a wheel and pinion worked with the hand. In other
cases, large bellows like those of the blacksmith, furnished with a wide
nozzle, are made to act in a similar way with the fanners. But these are
merely temporary expedients for small mines. A very slight circulation
of air can be effected by propulsion, in comparison of what may be done
by exhaustion; and hence it is better to attach the air-pipe to the
valve of the bellows, than to their nozzle.
Ventilation of collieries has been likewise effected on a small scale,
by attaching a horizontal funnel to the top of air-pipes elevated a
considerable height above the pit mouth. The funnel revolves on a pivot,
and by its tail-piece places its mouth so as to receive the wind. At
other times, a circulation of air is produced by placing coal-fires in
iron grates, either at the bottom of an upcast pit, or suspended by a
chain a few fathoms down.
[Illustration: 856]
Such are some of the more common methods practised in collieries of
moderate depth, where carbonic acid abounds, or where there is a total
stagnation of air. But in all great coal mines the aërial circulation is
regulated and directed by double doors, called main or bearing doors.
These are true air-valves, which intercept a current of air moving in
one direction from mixing with another moving in a different direction.
Such valves are placed on the main roads and passages of the galleries,
and are essential to a just ventilation. Their functions are represented
in the annexed _fig._ 856., where A shows the downcast shaft, in which
the aerial current is made to descend; B is the upcast shaft, sunk
towards the rise of the coal; and C, the dip-head level. Were the mine
here figured to be worked without any attention to the circulation, the
air would flow down the pit A, and proceed in a direct line up the rise
mine to the shaft B, in which it would ascend. The consequence would
therefore be, that all the galleries and boards to the dip of the pit A,
and those lying on each side of the pits, would have no circulation of
air; or, in the language of the collier, would be laid dead. To obviate
this result, double doors are placed in three of the galleries adjoining
the pit; viz., at _a_ and _b_, _c_ and _d_, _e_ and _f_; all of which
open inwards to the shaft A. By this plan, as the air is not suffered to
pass directly from the shaft A to the shaft B, through the doors _a_
and _b_, it would have taken the next shortest direction by _c d_ and _e
f_; but the doors in these galleries prevent this course, and compel it
to proceed downwards to the dip-head level C, where it will spread or
divide, one portion pursuing a route to the right, another to the left.
On arriving at the boards _g_ and _h_, it would have naturally ascended
by them; but this it cannot do, by reason of the building or stopping
placed at _g_ and _h_. By means of such stoppings placed in the boards
next the dip-head level, the air can be transported to the right hand or
to the left for many miles, if necessary, providing there be a train or
circle of aerial communication from the pit A to the pit B. If the
boards _i_ and _k_ are open, the air will ascend in them, as traced out
by the arrows; and after being diffused through the workings, will again
meet in a body at _a_, and mount the gallery to the pit B, sweeping away
with it the deleterious air which it meets in its path. Without double
doors on each main passage, the regular circulation of the air would be
constantly liable to interruptions and derangements; thus, suppose the
door _c_ to be removed, and only _d_ to remain in the left hand gallery,
all the other doors being as represented, it is obvious, that whenever
the door _d_ is opened, the air, finding a more direct passage in that
direction, would mount by the nearest channel _l_, to the shaft B, and
lay dead all the other parts of the work, stopping all circulation. As
the passages on which the doors are placed constitute the main roads by
which the miners go to and from their work, and as the corves are also
constantly wheeling along all the time, were a single door, such as _d_,
so often opened, the ventilation would be rendered precarious or
languid. But the double doors obviate this inconvenience; for both men
and horses, with the corves, in going to or from the pit bottom A, no
sooner enter the door _d_, than it shuts behind them, and encloses them
in the still air contained between the doors _d_ and _c_; _c_ having
prevented the air from changing its proper course while _d_ was open.
When _d_ is again shut, the door _c_ may be opened without
inconvenience, to allow the men and horses to pass on to the pit bottom
at A; the door _d_ preventing any change in the aerial circulation while
the door _c_ is open. In returning from the pit, the same rule is
observed, of shutting one of the double doors, before the other is
opened.
If this mode of disjoining and insulating air-courses from each other be
once fairly conceived, the continuance of the separation through a
working of any extent, may be easily understood.
When carbonic acid gas abounds, or when the fire-damp is in very small
quantity, the air may be conducted from the shaft to the dip-head level,
and by placing stoppings of each room next the level, it may be carried
to any distance along the dip-head levels; and the furthest room on each
side being left open, the air is suffered to diffuse itself through the
wastes, along the wall faces, and mount in the upcast pit, as is
represented in _fig._ 842. But should the air become stagnant along the
wall faces, stoppings are set up throughout the galleries, in such a way
as to direct the main body of fresh air along the wall faces for the
workmen, while a partial stream of air is allowed to pass through the
stoppings, to prevent any accumulation of foul air in the wastes.
In very deep and extensive collieries more elaborate arrangements for
ventilation are introduced. Here the circulation is made active by
rarefying the air at the upcast shaft, by means of a very large furnace
placed either at the bottom or top of the shaft. The former position is
generally preferred. _Fig._ 834. exhibits a furnace placed at the top of
the pit. When it surmounts a single pit, or a single division of the
pit, the compartment intended for the upcast is made air-tight at top,
by placing strong buntons or beams across it, at any suitable distance
from the mouth. On these buntons a close scaffolding of plank is laid,
which is well plastered or moated over with adhesive plastic clay. A
little way below the scaffold, a passage is previously cut, either in a
sloping direction, to connect the current of air with the furnace, or it
is laid horizontally, and then communicates with the furnace by a
vertical opening. If any obstacle prevent the scaffold from being
erected within the pit, this can be made air-tight at top, and a brick
flue carried thence along the surface to the furnace.
The furnace has a size proportional to the magnitude of the ventilation,
and the chimneys are either round or square, being from 50 to 100 feet
high, with an inside diameter of from 5 to 9 feet at bottom, tapering
upwards to a diameter of from 2-1/2 feet to 5 feet. Such stalks are made
9 inches thick in the body of the building, and a little thicker at
bottom, where they are lined with fire-bricks.
The plan of placing the furnace at the bottom of the pit is, however,
more advantageous, because the shaft through which the air ascends to
the furnace at the pit mouth, is always at the ordinary temperature; so
that whenever the top furnace is neglected, the circulation of air
throughout the mine becomes languid, and dangerous to the workmen;
whereas, when the furnace is situated at the bottom of the shaft, its
sides get heated, like those of a chimney, through its total length, so
that though the heat of the furnace be accidentally allowed to decline
or become extinct for a little, the circulation will still go on, the
air of the upcast pit being rarefied by the heat remaining in the sides
of the shaft.
[Illustration: 857 858]
To prevent the annoyance to the onsetters at the bottom, from the hot
smoke, the following plan has been adopted, as shown in the wood-cut,
_fig._ 857., where _a_ represents the lower part of the upcast shaft;
_b_, the furnace, built of brick, arched at top, with its sides
insulated from the solid mass of coal which surrounds it. Between the
furnace wall and the coal-beds, a current of air constantly passes
towards the shaft, in order to prevent the coal catching fire. From the
end of the furnace a gallery is cut in a rising direction at _c_, which
communicates with the shaft at _d_, about 7 or 8 fathoms from the bottom
of the pit. Thus the furnace and furnace-keeper are completely disjoined
from the shaft; and the pit bottom is not only free from all
encumbrances, but remains comfortably cool. To obviate the
inconveniences from the smoke to the banksmen in landing the coals at
the pit mouth, the following plan has been contrived for the Newcastle
collieries. _Fig._ 858. represents the mouth of the pit; _a_ is the
upcast shaft, provided with a furnace at bottom; _b_, the downcast
shaft, by which the supply of atmospheric air descends; and _d_, the
brattice carried above the pit mouth. A little way below the
settle-boards, a gallery _c_, is pushed, in communication with the
surface from the downcast shaft, over which a brick tube or chimney is
built from 60 to 80 feet high, 7 or 8 feet diameter at bottom, and 4 or
5 feet diameter at top. On the top of this chimney a deal funnel is
suspended horizontally on a pivot, like a turn-cap. The vane _f_, made
also of deal, keeps the mouth of the funnel always in the same direction
with the wind. The same mechanism is mounted at the upcast shaft _a_,
only here the funnel is made to present its mouth in the wind’s eye. It
is obvious from the figure, that a high wind will rather aid than check
the ventilation by this plan.
The principle of ventilation being thus established, the next object in
opening up a colliery, and in driving all galleries whatever, is the
_double mine_ or double _headways course_; on the simple but very
ingenious distribution of which, the circulation of air depends at the
commencement of the excavations.
[Illustration: 859]
The double headways course is represented in _fig._ 859., where _a_ is
the one heading or gallery, and _b_ the other; the former being
immediately connected with the upcast side of the pit _c_, and the
latter with the downcast side of the pit _d_. The pit itself is made
completely air-tight by its division of deals from top to bottom, called
the brattice wall; so that no air can pass through the brattice from _d_
to _c_, and the intercourse betwixt the two currents of air is
completely intercepted by a stopping betwixt the pit bottom and the end
of the first pillar of coal; the pillars or walls of coal, marked _e_,
are called stenting walls; and the openings betwixt them, walls or
thirlings. The arrows show the direction of the air. The headings _a_
and _b_ are generally made about 9 feet wide, the stenting walls 6 or 8
yards thick, and are holed or thirled at such a distance as may be most
suitable for the state of the air. The thirlings are 5 feet wide.
When the headings are set off from the pit bottom, an aperture is left
in the brattice at the end of the pillar next the pit, through which the
circulation betwixt the upcast and downcast pits is carried on; but
whenever the workmen cut through the first thirling No. 1., the aperture
in the brattice at the pit bottom is shut; in consequence of which the
air is immediately drawn by the power of the upcast shaft through that
thirling as represented by the dotted arrow. Thus a direct stream of
fresh air is obviously brought close to the forehead where the mines are
at work. The two headings _a_ and _b_ are then advanced, and as soon as
the thirling No. 2. is cut through, a wall of brick and mortar, 4-1/2
inches thick, is built across the thirling No. 1. This wall is termed a
stopping; and being air-tight, it forces the whole circulation through
the thirling No. 2. In this manner the air is always led forward, and
caused to circulate always by the last-made thirling next the forehead;
care being had, that whenever a new thirling is made, the last thirling
through which the air was circulated, be secured with an air-tight
stopping. In the woodcut, the stoppings are placed in the thirlings
numbered 1, 2, 3, 4, 5, 6, and of consequence the whole circulation
passes through the thirling No. 7., which lies nearest the foreheads of
the headings _a_, _b_. By inspecting the figure, we observe, that on
this very simple plan, a stream of air may be circulated to any required
distance, and in any direction, however tortuous. Thus, for example, if
while the double headways course _a_, _b_, is pushed forward, other
double headways courses are required to be carried on at the same time
on both sides of the first headway, the same general principles have
only to be attended to as shown in _fig._ 860., where _a_ is the
upcast, and _b_ the downcast shaft. The air advances along the heading
_c_, but cannot proceed further in that direction than the pillar _d_,
being obstructed by the double doors at _e_. It therefore advances in
the direction of the arrows to the foreheads at _f_, and passing through
the last thirling made there, returns to the opposite side of the double
doors, ascends now the heading _g_ to the foreheads at _h_, passes
through the last-made thirling at that point, and descends, in the
heading _i_, till it is interrupted by the double doors at _k_. The
aerial current now moves along the heading _l_, to the foreheads at _m_,
returns by the last-made thirling there, along the heading _n_, and
finally goes down the heading _o_, and mounts by the upcast shaft _a_,
carrying with it all the noxious gases which it encountered during its
circuitous journey. This wood-cut is a faithful representation of the
system by which collieries of the greatest extent are worked and
ventilated. In some of these, the air courses are from 30 to 40 miles
long. Thus the air conducted by the medium of a shaft divided by a
brattice wall only a few inches thick, after descending in the downcast
in one compartment of the pit at 6 o’clock in the morning, must thence
travel through a circuit of nearly 30 miles, and cannot arrive at its
reascending compartment on the other side of the brattice, or pit
partition, till 6 o’clock in the evening, supposing it to move all the
time at the rate of 2-1/2 miles per hour. Hence we see that the _primum
mobile_ of this mighty circulation, the furnace, must be carefully
looked after, since its irregularities may affect the comfort, or even
the existence of hundreds of miners spread over these vast subterraneous
labyrinths. On the principles just laid down, it appears, that if any
number of boards be set off from any side of these galleries, either in
a level, dip, or rise direction, the circulation of air may be advanced
to each forehead, by an ingoing and returning current.
[Illustration: 860]