AN ARTIFICIAL COAL MINE
Those countries which are blessed with a plentiful supply of coal are
periodically shocked and saddened by a terrible calamity--an explosion
in one of the mines, in which often scores of poor fellows lose their
lives, and hundreds of widows and orphans find themselves without a
breadwinner. One has only to recall that heart-rending calamity of the
Courrieres mines in France, where over a thousand lives were lost, to
realise how important is the question of the cause and the cure of the
colliery explosion.
It used to be thought a settled matter that these were due to the
accidental ignition of a gas called, scientifically, "methane," but by
the miners "fire-damp." This undoubtedly does collect in many mines, and
since it is much the same as the domestic coal-gas (indeed methane forms
the bulk of coal-gas) it is not surprising that the explosions were
attributed to it. At times shots were fired, to blast down the coal, and
although the greatest precautions are taken to prevent any accident
resulting, it seems certain that explosions have occasionally followed
the firing of shots. But still more dangerous is the adventurous miner
who, for some reason, opens his safety lamp. It is lit for him before he
enters the workings, and locked up, so that, theoretically, he cannot
tamper with it; but it has to be a cleverly devised lock that cannot be
picked in some way, and with the carelessness born of long immunity from
accident these are got open sometimes, with, it may be, disastrous
results.
Even a spark struck from a miner's pick may ignite the gas; or a spark
from some electrical machine used in the mine. That is one of the
reasons why electrical apparatus is suspect in colliery matters and
machines worked by the less convenient and more costly means of
compressed air are preferred.
In some such manner the fire-damp is ignited, and then there follows the
fiery blast, which, sweeping through the narrow galleries and passages
which constitute the workings, simply licks up the life of the men whom
it encounters. Others, in byways and sheltered corners, escaping the
burning cloud of flame, are poisoned by the deadly fumes of carbon
monoxide which it leaves when its force is spent. While others,
perchance the most unfortunate of all, are saved for a time, but, being
imprisoned by falls from the roof and walls, die a lingering death of
hunger and slow suffocation. A colliery explosion is one of the
ghastliest events imaginable, the only relief from which is the noble
heroism with which the survivors, from the mine managers to the humblest
workmen, crowd round the pit-mouth, eager to risk their own lives for
the faint chance of saving some below. Not infrequently these brave
volunteers only share the fate of the men they would rescue.
Now all that, as I have said, used to be put down to the effect of the
fire-damp. But it dawned upon men's minds some years ago that the damage
seemed to be out of proportion to the power of the gas. Modern mines are
well ventilated by large fans, which impel great volumes of air through
all the workings. The air currents are cunningly guided by partitions or
"brattices," so that every nook and corner shall be scoured out by the
plentiful draught of pure fresh air. Consequently the amount of
explosive gas which can collect in any one place is but small. How,
then, can so small a volume of gas do so large an amount of damage?
Coupled with this was the fact that explosions take place in flour
mills, where there is no gas, and experimenters had found in their
laboratories that almost any burnable substance, _if ground up finely
enough_ and blown into a cloud, would explode. Coal-dust would naturally
do this. Indeed anyone throwing the dust from the bottom of the
coal-shovel upon a fire will see for himself how, quickly such dust
will burn, and, as has been pointed out in an earlier chapter, an
explosion is but rapid burning.
So the blame was largely transferred from the shoulders of the fire-damp
to those of the clouds of coal-dust which collect throughout the
workings of a mine.
But then a difficulty arose from the fact that there is dust in all
mines, yet some districts are quite free from explosions. And such
districts are those where there is little or no fire-damp. These two
facts seem to be explainable in one way, and in one way only. It must be
that the gas first of all explodes feebly, and so, stirring up the dust
lying along the roads and passages, prepares the way for the powerful,
deadly explosion of coal-dust which follows.
But that was only a guess, and the matter was of such importance that it
needed something more certain than mere assumption. So the Mining
Association of Great Britain decided to have a series of experiments
which should settle once and for all what part the coal-dust played in
these catastrophes, and how best they could be prevented.
It was at first thought that an old mine might be utilised for the
experiments, but there was the difficulty that such always become wet
after work has ceased in them, and so the dust would not behave
normally. Moreover, the work would be extremely dangerous and the
results difficult to observe. Then a culvert was suggested built of
concrete, partly buried in the ground, but that too was dismissed.
Finally it was decided to make an imitation mine of steel, using old
boiler shells with the ends taken out.
The sum of L10,000 was subscribed for the purpose by the coal-owners of
Great Britain, and the great work was carried out at Altofts, in
Yorkshire, close to a colliery where a terrible disaster occurred in
1886.
Here the great tube or gallery was built. Roughly the shape of a letter
L, one leg is over 1000 feet long, while the other is 295
feet. The longer leg is 7-1/2 feet in diameter and the shorter 6 feet.
At the end of the shorter part a large fan is installed which can force
50,000 to 80,000 cubic feet of air per minute through the structure, so
producing the conditions of a well-ventilated mine. The shorter length
has several sharp turns in it for the purpose of breaking the force of
the explosion along that part, and so shielding the fan from damage,
while a tall chimney is provided there, so that, the door being shut to
cut off the fan, the gases from the explosion can find a harmless way
out.
Inside the tube, shelves are fixed along the sides so as to reproduce
the effect of the timbering in a real mine, upon the beams of which the
dust finds lodgment. Props were put up too, just as they would be in the
real mine. Everything, in fact, was done to make the place as perfect a
replica as possible of actual underground workings.
And then, added to this huge and costly structure, was an outfit of
scientific instruments worthy of the important investigations which were
to be carried on.
To grasp the purpose and working of these we need to remind ourselves of
the aims and intentions of the experiments. First of all it was desired
to find out how various quantities and qualities of coal-dust behaved.
The dust was laid along the floor of the tube and along the shelves. A
small gun fired at some point in the tube raised a cloud of this dust
just as the gas explosion in the real mine would do. Then another gun
was fired to explode the dust-cloud. So far all is quite simple and
easy. But to do that would be of no value without the means of finding
out exactly what resulted from the explosion. And that is the function
of the instruments.
To commence with, there is the great wave or tide of force or pressure
which surges along the gallery immediately the cloud bursts into flame.
How fast does that wave travel? How long is it after the explosion
before the shattering effects of it are felt a hundred yards away? To
solve that problem electrical contact-breakers are fixed at intervals of
fifty yards along the gallery. Each of these consists of a cylinder with
a piston inside it something like, shall we say, a cycle pump. The
piston, held down normally by a spring, is blown upwards by the force of
the explosion. The spring is adjustable, and so it can be arranged that
the feeble force of the gun cannot lift the piston, but the more
powerful coal-dust explosion which follows can.
Thus when the explosion takes place these contact-breakers are operated
in succession. The one nearest the seat of the disturbance is operated
first; next the one fifty yards farther away; then the one a hundred
yards away, and so on. The moments when they work will tell the speed at
which the blast travels along the gallery. But it travels with great
speed, and so to measure and record the exact moment when each
contact-breaker is moved is a matter of no little difficulty.
Electricity, however, makes this, like so many other things,
comparatively easy.
There is an apparatus used in astronomical observatories called a
chronograph, which registers, within a small fraction of a second, the
moment when a star seems to pass across a wire in the "transit circle,"
the telescope by which the positions of stars are determined and the
exact time kept. The observer sits with his eye to the telescope,
watching the apparent movement of the star. In his hand he holds a small
"push," pressure on which by his fingers operates a minute pricker,
which acts upon a moving strip of paper. The paper travels along with
the utmost steadiness and regularity, while a clock drives a sharply
pointed pricker on to it every two seconds. Thus the clock marks out the
paper into lengths, each of which represents two seconds. But the other
pricker, worked electrically by the observer's hand, also makes its mark
upon the paper, and so, while the regular marks indicate intervals of
two seconds, each irregular one marks the time of a transit or passing
of a star across the wire. An examination of the strip subsequently
enables the times of a transit to be seen with great accuracy, from the
position of the corresponding mark between two of the _regular_ marks.
And the same principle was applied to the circuit-breakers of this
artificial mine. Normally, current flows through the circuit-breaker,
but the lifting of the piston breaks the circuit (whence the name of the
contrivance), and that breaking of the circuit and consequent cessation
of the current operates the chronograph. By a cleverly constructed
device, the details of which are too complicated to set out here, each
circuit-breaker in turn makes its mark on the same strip, so that the
distances apart of these marks show the time taken by the force of the
explosion to travel fifty yards. Meanwhile the clock goes on making its
regular marks (in this case every half-second), so that they form a
scale by which the other intervals can be measured very exactly.
The chronograph used here is more accurate than that in use at Greenwich
Observatory, the reason being that in this case the recording currents
are sent mechanically by the contact-breakers operated by the explosion
itself, while in the case of the astronomer the human element comes in.
To watch a moving speck of light and to tell exactly when it crosses a
fine line is by no means easy, and so to tell the time within a tenth of
a second, is about the limit of possible accuracy. The instrument we
have been referring to, however, can register the time which a gaseous
wave moving 3000 feet per second takes to travel fifty feet. In other
words, the circuit-breakers can be operated so fast that when only a
sixtieth of a second intervenes between the action of one and that of
the next the chronograph can duly record the fact.
The records of the chronograph can be made in two ways: one by a pen on
a piece of paper tape, and the other by a scratch on a piece of smoked
paper.
So by that means the progress of the "force" of the explosion can be
measured. It is necessary also to time the movement of the "heat" of the
explosion, for the two may not travel together, and the difference
between them may let in some light as to the nature and behaviour of the
explosion. So for this second purpose a second set of circuit-breakers
are used. Each of these consists of a strip of thin tinfoil stretched
across the gallery. Being placed edgeways to the moving current of gas,
the force of the explosion has no effect upon it, but the heat instantly
melts it. Normally, current flows through the strip, and so the melting
is signalised by the cessation of the current, which event is recorded
by the chronograph.
Thus the speeds at which the force and the heat of the explosion travel
are ascertained. Another important fact which needs to be found is the
amount of the force, or the pressure, at different points. For this
purpose pressure-gauges can be connected to the gallery at the desired
spots by means of flexible tubes. This flexible tube is necessary in
order that the vibration of the steel shell, due to the explosion, shall
not be communicated to the instrument. The pressure, finding its way
along the flexible pipe, raises a piston against the force of a spring,
and the distance to which it is raised forms, of course, a measure of
the pressure inside the gallery at the point to which the tube is
connected. The pressure is recorded by the action of the piston in
moving a style which just touches against the surface of a moving paper.
There are three styles in all marking this paper. The first is the one
just mentioned. The second is held down on to the paper by an
electro-magnet energised by current flowing through a fine wire
stretched across the gallery just where the explosion originates. This
fine wire is broken at the moment of the explosion, whereby the current
is cut off and the style raised. It therefore makes its mark until the
moment the explosion occurs, and then leaves off. The end of that line,
therefore, shows the time of the explosion. Meanwhile the first style is
drawing a straight line, but as soon as the pressure begins to be felt
by the pressure recorder this style moves and the line slopes upward.
Upward it goes as the pressure increases, until it has reached its
height, after which it descends, until the style is drawing a straight
line once more. Thus the rise and fall of the line represents the rise
and fall of the force of the explosion.
Then comes the matter of time. How soon after the explosion occurred did
the pressure begin to be felt? How long did it take to reach its maximum
and how long to die out again? These questions need answers which the
apparatus so far described does not give. True, the speed of the paper
may be known approximately, but all that I have described will occur
within the space of a fraction of a second, and it is difficult to tell
the speed of the paper with sufficient accuracy. Therein we see the
purpose of the third style. It is attached electrically to the
"tenth-of-a-second time-marker." This consists of a weight suspended at
a height. The force of the explosion lets it drop. The moment it starts
to fall it causes the style to make a mark on the paper. When it has
fallen a certain distance the style makes another mark. And the distance
that the weight falls between the making of the two marks is so adjusted
that the space between them on the chart represents exactly a tenth of a
second. Thus a scale is formed upon the chart by which the other times
can be measured. There is the line terminating at the moment of
explosion; the straight line changing into an up-and-down curve,
representing the time and the variation of the pressure; finally there
are the two marks representing a tenth of a second by which the other
marks recorded upon the chart can be interpreted.
But the mere pressure and velocity of the explosion form but a part of
the knowledge desired. How the explosion is formed, whether or not the
coal-dust is burnt up entirely, whether, indeed, it be the dust itself
which burns or coal-gas given off by the dust under the heat of the
preliminary explosion, what the gas is which is left by the explosion at
various stages--these are important things to be known, and they can
only be ascertained by taking samples of the gases in the gallery at
different moments during and after the explosion. To obtain these
samples bottles are used, but the question is how to get them filled at
just the right time. Into the shell of the gallery holes are drilled,
and to these the metal bottles or flasks are screwed, a pipe leading
from the mouth of each bottle well in towards the centre of the gallery.
The end of this tube is closed by a cap of glass above which there
stands poised a little hammer. Controlling the hammer is an electrical
device called a "contact-maker," so arranged that just at the desired
moment the hammer falls, breaking the glass, and admitting a sample of
the gas in the gallery, the bottle and its tube having previously had
the air exhausted from them, so that on the glass being broken the gas
is sucked in.
At the same moment a weight falls, attached to the end of a cord, and
this, on reaching the end of its tether, closes the end of the tube, and
the sample is imprisoned until such time as the bottle can be
disconnected and taken away to the laboratory for its contents to be
analysed.
The contact-makers are of two kinds. In one the pressure of the
explosion raises a piston which completes a circuit allowing current to
flow through the very fine wire which prevents the fall of the hammer.
This fine wire being fused by the current, the hammer falls and does its
work. The other kind, which are used when the force of the explosion is
not enough to raise a piston, is operated by one of the tinfoil
circuit-breakers. A magnet, being energised by current passing through
the foil, holds up a curved bar over two cups of mercury. Broken by the
heat of the explosion, the foil cuts off this current, de-energises the
magnet, and allows the bar to fall with its ends in the mercury. This
completes another circuit, permitting current to pass to the fine wire,
whereby the hammer is released. By connecting a bottle to a
contact-maker at a distance the sample can be obtained at any desired
period of the explosion. If, for instance, the sample is to represent
the immediate products of combustion, it is placed near to the
contact-maker. Then the sample is drawn in practically at the moment of
explosion. If, on the other hand, it is the after-damp that is to be
sampled, then the bottle would be connected to a contact-maker a long
way from the seat of the explosion, with the result that its glass cap
would not be broken until some considerable time had elapsed after the
explosion has passed the bottle. The time also during which the bottle
is drawing in its sample can be adjusted by varying the length of the
cord to which the weight is attached.
And last of all must be mentioned the employment of a kinematograph,
capable of taking twenty-two photographs per second, for observing the
effects at the ends of the gallery (see illustrations).
Thus records are obtained of the force and heat of the explosion, its
mechanical and thermal effects upon the walls of the gallery, or, if it
were in a real pit, the effects which it would have in shaking and in
heating the workings, and the men labouring in them. This and the
analysis of the gases producing and produced by the explosion, derived
from the contents of the bottles, give sound data upon which can be
built up reliable theories as to the nature of colliery explosions and
the way to prevent them, results which could be obtained in no other
way. No one can help being struck with the thoroughness and ingenuity of
the means adopted to these ends, and it is no exaggeration to say that
it is a splendid example of thoroughly scientific methods applied to an
important industrial investigation. It will be interesting to conclude
this account with a brief mention of some of the results to which these
painstaking efforts have led.
First in importance the fact is placed beyond doubt that coal-dust,
which in bulk will only burn slowly, will, when well mixed with air,
explode. And no combustible gas need be present to aid in the explosion.
The dust-raising gun, by blowing some dust into a cloud which was
ignited by the second gun, caused an explosion powerful enough to do all
the damage experienced in the most disastrous natural explosions. So it
is practically certain that the function of the gas is but that of the
first gun, to raise the cloud of dust.
A typical experimental explosion may be briefly described. On the
cloud-raising gun being fired a small cloud of dust was driven out of
the ends of the gallery, even that end at which the fan was blowing air
_in_. In other words, the current of air was checked, even reversed, by
the preliminary shock. This cloud was, of course, shown by the
kinematograph.
Then when the second gun was fired, and the real coal-dust explosion
occurred, there was first a cloud of dust shot out larger than the other
one, to be followed by a cloud of flame 180 feet long. These also were
recorded by the kinematograph. The sound was heard seven miles away.
Pressures as high as 92 lb. per square inch were recorded, and the force
of the blast was found to travel well over 2000 feet per second.
In many cases, strange to say, the effects were very slight at the seat
of the disturbance, the force seeming to increase as the wave travelled
along the gallery. Probably the dust had not time to burn completely but
only partially at the first onset. Where props or timbers checked the
flow of the flaming gases there the damage was most, for no doubt the
eddies caused the air and coal to be particularly well mixed at such
points. An encrustation of coke was found on the sides and the timbers
after all was over, probably because there was not sufficient air to
burn all the dust, and some was only heated into coke to be deposited on
the nearest surface, where the tarry matters would make it stick.
Finally, the most important, perhaps, of all, it was demonstrated that
an admixture of stone-dust with the coal-dust made it non-inflammable.
If a small zone were treated in this way, stone-dust being mingled with
the other, the explosion became stifled at that point. True, the
poisonous after-damp swept on beyond, so that men there might have been
poisoned by it, but the stone zone would certainly save them from the
direct effects of the blast. If, however, stone-dust be mingled with
coal-dust all along the gallery, then no explosion at all would occur,
again proving that it is the coal-dust which does the damage.
In the colliery adjoining the experimental gallery this plan had been in
use for years. Soft shale is ground to fine powder, and is sprinkled
wherever coal-dust has collected. It is just strewn by hand, giving the
workings the appearance of having been roughly whitewashed. And since
that has been done there has been no explosion in that pit. The
experiments showed beyond doubt that that was no chance occurrence. They
showed that in some way not thoroughly understood this addition of
stone-dust renders the coal-dust harmless. It may be that it merely
dilutes it. It may be that in some way it takes some of the heat and so
prevents the coal particles becoming hot enough. It may be that, being a
little heavier, it checks the formation of the dust-cloud. However that
may be, there is no doubt now that stone-dust is the salvation of the
miner so far as explosions are concerned.
Water sprinkled upon the coal-dust, by laying it and keeping it from
forming a cloud, has the same effect, but it is less convenient, for the
simple reason that water evaporates, while stone-dust stays where it is
put.