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 Courrières 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 £10,000 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.