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 "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 extreme 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 BF, Sweep the faces C E, FE, 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 66 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 sub- · division 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

known as "blowers" are liable to occur, because the discharge of gas from these is so sudden and so enormous as to overpower the most plentiful and perfect ventilation. In nearly all coals there is a risk, and in some a certainty, of meeting with these blowers, and for perfect security we must find some means of removing the danger arising from them. This is done by employing what are known as "safety-lamps," that is, lamps which may be carried into an explosive mixture of carburetted hydrogen and air without firing the compound. The principle on which they depend was discovered independently about the same time by Sir H. Davy and George Stephenson, and is as follows:-It was found that if a lamp or candle be enciosed in an envelope of fine wire gauze, containing not less than 600 holes to the square inch, any explosions which take place within the envelope cannot be communicated to the gas outside, and that the flame cannot pass through the gauze except under pressure.* In Davy's original lamp the light is simply enclosed in a cylinder of this gauze, but this arrangement has a somewhat feeble illuminating power. Modifications have been made by the introduction of glass, which give a better light; and other contrivances have been proposed for increasing the draught, and thus obtaining a similar result. The principal forms of safety-lamp are well described in the "Rudimentary Treatise on Coal and Coal Mining," by Mr. Warrington Smyth (Lockwood & Co., 1872).

Unluckily, when perfect ventilation and an efficient safetylamp have been provided, the colliery manager's cares are not at an end. The working collier is proverbially reckless, and nothing can prevent him from opening his lamp, if he can, to get a better light for his work, to light his pipe, or even sometimes from foolhardiness. Lamps are locked before being given into the men's hands, and then the men carry keys. Lamps are constructed which go out directly they are opened, and then the men take down lucifers and light them again. Lamps have been devised which are locked with a plug of lead, on which a device is punched, and which cannot be opened without breaking the plug; and some such troublesome precaution, it seems, must be adopted, if tampering with the lamp is to be put an end to. The latest contrivance is a lamp which is closed by a steel spring, and can only be opened by the action of a very powerful magnet on the spring. The magnet is kept in the custody of the head manager, and as it is obliged to be a far more powerful one than the colliers are likely to be able to obtain, this plan, if it succeed in other respects, seems likely to be effective.

For an explanation of the physical reason of these facts, see Tyndall's "Heat as a Mode of Motion," p. 240.

A probable cause of accidents which we can only hint at here is the spontaneous combustion of the waste coal left in the goaf; and it is also highly likely that some explosions have been caused by blasting in fiery pits.

It is further to be noted, that even the most perfect safetylamp requires occasionally the utmost caution in using it. In a fiery atmosphere the combustion and explosion of gas within the lamp sometimes raises the gauze to a red-heat, and in this way sets light to the explosive mixture outside. It has also been repeatedly proved by experiment that no lamp is safe in a strong current of air. The velocity necessary to cause an explosion varies with different forms of lamp, but all that have been yet devised blow up sooner or later, if the force of the draught in which they are placed is gradually increased.*

Science has therefore still something further to do for the collier in the matter of lighting him at his work; and the most promising quarter, perhaps, to which the would-be inventor can turn his attention is the electric light. If this could be produced cheaply and in a portable form, we should have in it all the conditions of perfect safety; for the light may be completely cut off from the explosive atmosphere by surrounding it with a glass globe, and a cage of a few iron bars would guard against any risk of fracture to the glass. Even now it seems that this source of light might be usefully employed in these exceptional cases, like the first opening out of a colliery after an explosion, when much of the work has to be done in the dark. A beam of parallel rays sent down the shaft by an electric lamp at the top would have intensity enough to allow of its being reflected by mirrors into the workings, and would make the task of beginning to open out a wrecked colliery easier and more expeditious. And as soon as a cheap galvanic battery is invented, there seems to be no reason why we should not light our collieries with a brilliancy undreamed of now, and at the same time get rid of all risk of explosion.

We have not yet said anything about the means of detecting the presence of fire-damp, and since it is as true in a mine as elsewhere that to be forewarned is to be forearmed, this part of our subject must not be passed over. It has been noticed that many serious explosions have been preceded by rapid falls of the barometer, and it is not hard to imagine how sudden diminution of atmospheric pressure might well affect so light and easily moved a gas as carburetted hydrogen. Every colliery ought therefore to be furnished with a good barometer, and its readings constantly noted; and whenever a rapid fall takes

Among the latest of these experiments are some made at the Barnsley Gas Works. See "Mining Journal," 1867, p. 530.