Carbonic acid is known among the colliers as "black-damp," or "after-damp."

We must also note that carburetted hydrogen is very light, not much more than one-half as heavy as common air, and in spite of the general property of gases to diffuse themselves uniformly without reference to their relative density, it tends, in virtue of its low specific gravity, to accumulate in the higher parts of a mine, in bell-shaped hollows in the roof, and in the stagnant atmosphere of the "goaf," or abandoned workings.

Such then are the deadly qualities of this subtle enemy, which exists often in enormous quantity, pent up in the body of the coal itself. How it came there is a question to which it is not easy to give a definite answer. The same gas, however, is produced by the decomposition of dead plants in marshy places, and we must content ourselves with the vague statement that carburetted hydrogen has been generated by chemical reaction out of the vegetable matter of which coal is made up.

But whatever be the exact cause of its presence, there it is, locked up in a highly condensed state, and ready to burst forth the moment the pressure which holds it back is removed by the operation of cutting into the coal.

In some beds the gas seems to be uniformly disseminated through the whole body of the seam, and in these cases it is most striking to stand in front of a newly bared face of coal, and hear it rushing out, like bees out of a hive, as the colliers say, with an incessant sputtering and hissing, while small bits of coal keep flying off with a sharp cracking sound, as the imprisoned gas bursts the cells in which it is confined. Gas of this sort is comparatively harmless, as it gives warning of its presence, and can be dealt with and subdued by methods to be described further on. Far more serious are these cases when a single blow of a pick taps an enormous reservoir of pent up gas, and an outburst follows, with a roar like that produced by the escape of high-pressure steam, so suddenly and in such quantity, that it overpowers all the ordinary precautions, and nothing can be done but to leave it till it has exhausted itself or considerably abated. Such discharges are known as "blowers;" they occur very generally in the neighbourhood of faults or dislocations, and it seems likely that the gas finds its way up the rent in the strata, from great depths, where it exists in a still higher state of compression than in the coal itself. Outbursts of this nature sometimes force their way through the floor of the mine, in. which they tear long rents, as if the solid rock were so much parchment.* Some of these

See "Transactions of the Milland Institute of Mining Engineers," vol. ii. pp. 155, 189.

sudden discharges blow themselves off in a short time: others go on for years without showing any signs of abatement, and the gas from them has been collected, and used in those parts of the mine where present custom looks upon it as safe to employ naked lights.

The reader will now realise the danger which the collier stands always face to face with: gas to some extent is almost always present, and should it be mixed with air in the proper proportions and come in contact with a naked light, an explosion more or less severe immediately follows.

The methods of coping with this difficulty may be grouped under two main heads. One will include the processes of ventilation, by which the gas is diluted with common air to such an extent as to render it harmless: the other will take in all the contrivances for shielding those lights which must be used, so as to prevent their coming in contact with any explosive compound that ventilation may have failed to

remove.

First of ventilation-absolutely necessary in every case to supply breathing air to the miner, and doubly needful where the earth is ever pouring out a deadly gas, which lurks in each sheltered spot, and can only be driven out by sweeping every nook and corner by something like a hurricane of pure air.

Every well-ordered coal mine is now supplied with at least two separate shafts, the size of which depends on its area, 12 to 15 feet clear diameter within the brickwork being not too large if the workings are very extensive. Down one of these, known as the "downcast" shaft, pure air is constantly passing, and is conducted through the workings, driving before it the contaminated atmosphere, and in the end forcing it up the other shaft, which serves for an outlet, and is called the “ upcast" shaft. The air taken in goes by the name of the “intake" current, and the foul compound discharged from the upcast is called the "returns." It was at one time customary to carry all the air in one body through the windings and turnings of the workings, but by this method the current became too much contaminated, and its velocity too much reduced before it reached the end of its long journey, to allow of its producing any useful result in the parts of the mine most distant from the downcast shaft: accordingly the intake air is now split" into separate currents, varying in number according to the size of the mine, before it begins to run its course; each of these ventilates a separate district, and in the end all unite and are discharged through the upcast.

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The passage of the air through the workings is regulated by barriers known as "stoppings;" by doors, often double like a canal lock, which allow the miner to pass through, but keep

back the air current; or in some cases simply by hanging heavy cloths across an opening through which it is wished to prevent the passage of the air. Sometimes, when the ventilating current has to cross one of the main roads which it would be inconvenient to interrupt by doors, the air is carried across in a distinct passage over the roof of the gallery.

Thus there is constantly going on a process which does for the mine exactly what the functions of breathing and the circulation of the blood do for the human body; and we must next see by what means the circulation is kept going in the case of the mine.

The methods usually employed are twofold: sometimes a large fan, turned by machinery, is placed at the top of the upcast shaft, to create an upward draught; sometimes a furnace is kept burning in that shaft, which rarifies the air, causes it to rise, and sucks a corresponding quantity to fill its place down the downcast.

The disadvantage of a furnace is that the returns may be so charged with fire-damp as to explode on coming in contact with the flame. To obviate this the foul air is carried through a gallery, known as a "dumb drift," which opens into the upcast some way above the furnace : even this precaution, however, is thought to have been ineffectual in some very fiery mines, and good authorities have given it as their opinion that there are cases where no precaution can render a furnace safe, and that a fan ought then to be employed. One great drawback to the use of a fan is, that the instant any accident happens to it, or the machinery which turns it, ventilation is at once stopped; whereas with a furnace, especially if it be placed at the bottom of the upcast shaft, and that shaft be deep, a long time will elapse before the heated column of air can cool down to the same temperature as the rest of the mine, and ventilation, to a certain extent, will go on after the furnace has gone out, should either accident or carelessness cause that to happen. Hence, when fans are used, it is very desirable to have at least two, so that if one be disabled the other may immediately take its place.

One or two other points connected with ventilation call for notice. Beds of coal very rarely lie perfectly flat; in most cases they are inclined, or "dip" at various angles to the horizon. Now, the small specific gravity of fire-damp tends to make it flow of itself towards the highest, or 66 rise parts of the mine, and the tendency should be taken advantage of to facilitate the escape of the gas. Hence it is desirable to put the upcast shaft as far to the rise as can be done. Further we have noticed the tendency of gas to accumulate in abandoned workings or "goafs," from which falls of the roof are liable to

force it out into the mine, or from which sudden decrease in the pressure of the surrounding air allows it to escape. This makes the "goaf" a constant source of danger, which should be entered or traversed as little as possible, and, as far as may be, cut off from communication with workings in progress. Putting these facts together, it is clear that the safest combination is to have the "goaf" in the rear and to the rise of the workings; and that, cæteris paribus, the safest way of getting the coal is that known as "long wall," by which all the mineral is extracted at the first working; and that the "pillar and stall" method,

by which portions are left to support the roof, and afterwards got out by a second working, is liable to objection, because it involves the carrying on of operations in the middle of a mass of abandoned workings, loaded with fiery gas. It is also clear that several advantages will be gained by placing the downcast shaft on the extrême dip of the mine, and making it the shaft by which the mineral is raised to the surface. For by this means the intake air drives the light fire-damp in the direction in which it naturally tends to move, and the coal may be run down to the shaft bottom by its own gravity.

Fig. 1 shows a plan of what may be called a coal mine under its simplest form, in which all these advantages have

been secured. D is the downcast, v the upcast shaft, and the bed of coal rises from D to V. The first thing done on beginning to work the mine is to drive out roads, D A, D B, as nearly level as possible, to the right and left of D up to the boundaries of the royalty; the roads A A', DV, B B', are then driven at right angles to A B, up to the boundary on the rise side the boundary on the dip side is A B. : The extraction of coal now begins, and the mineral is sliced off in long faces parallel to A'B', starting at the rise boundary, and working backwards to the dip. In this way the "goaf" is always behind and to the rise of the workings, and is traversed only by the air course EV, which may be bricked and completely isolated from the fiery district on either side. The intake air is split into two courses, which travel along DA C, D B F, Sweep the faces C E, F E, and pass along EV to the upcast. The coal as it is extracted is drawn along the levels CE, FE, and run down the incline ED to the drawing shaft D.

Very simple indeed this looks on paper, but unluckily in practice innumerable obstacles come in the way, when we attempt to carry out such a plan. First natural obstacles occur. The rise of the coal is not constantly in the same direction, and the seam is often traversed by lines of fracture, known as "faults" or "troubles," by which the bed has been broken across, and the part on one side raised or depressed relatively to the part on the other side. Commercial considerations give rise to still more serious difficulties. The subdivision of property often makes it difficult to obtain a tract of coal anything like as symmetrical as that in the sketch. The time taken in driving the preliminary roads or "straight work" would in a large colliery be very considerable; and adventurers do not like to see their money lying idle so long, and are tempted to obtain quicker returns by beginning to raise coal as near to the shaft as is safe. The position of the drawing shaft is mainly determined by the consideration that it must be as near as possible to the canal or railway that is to carry the coal to market, and it may well happen that this is not on the dip of the mine. Unless the mine be very large indeed, it will be impossible to obtain from a single face an output large enough to make the undertaking pay. These and other obstacles, too numerous to mention here, prevent us in practice from ever obtaining all the advantages possessed by the plan shown in Fig. 1, and corresponding compromises have to be made; but the general principles there illustrated are such as guide the engineer in the laying out of a colliery.

The precautions already described would render a mine, in which gas exuded uniformly and slowly from the coal, absolutely safe; but they do not alone suffice where the outbursts