But, as respects their introduction throughout the workings of a pit, the question is somewhat complex. It is apt to be the case, that if one precautionary measure be fully installed, another is neglected,—that when safety lamps are adopted for the entire operations of a mine, the ventilation is no longer a subject of the same attention; and unless there exist good local reason for it, it is obvious that the protection by wire gauze against present fire-damp is a less desirable kind of security than that of drowning the enemy in a full ventilating current, and sweeping him bodily away. Where the gas, however, is not merely given off continuously from the surfaces of freshly-cut coal, but bursts out from time to time in sudden blowers, the general use of safety lamps is imperative; and on such occasions, when for a short time the best ventilated workings may be "fouled," or rendered explosive, the lives of all in the pit will depend on the proper condition of the lamps, and on the obedience to discipline of those men who are interposed between the point of outburst and the exit to the surface. Similarly, in the working of pillars, where, with the movement of the ground, firedamp may exude either from the roof or floor, or may be forced by a fall from the magazine in which it has been collecting, safety lamps are indispensable. It commonly occurs, that although such may be the case in portions of a colliery, other parts, and especially the ordinary narrow work in whole coal, may be safely conducted with open lights. Here it will be necessary to fix on certain limits within which safety lamps alone are to be employed, and to make it a stringent rule that no naked light be allowed to pass beyond a definite point in the roads. In Fig. 19, the bords, on the north,

are worked with candles, the pillars, adjoining the goaf, with safety lamps; a special door is fixed upon as the place, beyond which no open light is allowed to be carried; and the course of the ventilating current, led backward and forward three times, as seen by the arrows in the figure, is so contrived as to guard against any communication of gas from the dangerous portions to the bords.

In no department of mining is a strict discipline and attention to orders so momentous as in this,-the question of lighting. The misplaced confidence, which is the result either of ignorance, of hardihood, or of long impunity, has led to the sacrifice of thousands of colliers, the innocent often suffering with the guilty; and among the most useful of the innovations of the governmental inspection is, that of giving authority to the code of rules to be established for every pit, and thus of protecting the majority of the men, the steadier workers, against the few reckless ones, who, choosing to act for themselves, steal in secret the luxury of their pipe, or some extra light, at the risk of their own and their comrades' lives.

CHAPTER XVII.

VENTILATION.

Ir needs no argument to impress on those who know the necessity of ventilating our public and private rooms, that it is in a high degree essential to take thought for the replacement of vitiated by fresh air in the low and often-complicated chambers of coal mines,

where many men and horses are engaged in hard work, and where numerous lights, with gunpowder smoke and dust, aid in contaminating the atmosphere. But, in the workings of a colliery, additional causes come into play; a slow, yet constant change takes place in the surface of the substances exposed to the air, and the general result is, the absorption of oxygen; a large amount of watery vapour requires removal; the poisonous gas, carbonic acid, is frequently given off; and, more commonly, the insidious fire-damp, or carburetted hydrogen, exudes from the surfaces of the bared coal, or sometimes bursts from it in violent jets. The amount of air required for the health and safety of the men will therefore vary much in different localities, according to these unequal conditions; and whilst, in some cases, the slightest movement of air may suffice to keep a small colliery salubrious, in fiery coals worked over a large area an actual whirlwind must be forced through the principal passages in order to sweep away the noxious exhalations.

Notwithstanding the undoubted phenomena of the diffusion of gases, their intermingling in the chambers and drifts of mines is only partial, and the specific gravity of the gaseous bodies is practically a very important guide in testing their presence, and enabling them to be dealt with. Thus, carbonic acid (CO2), with a specific gravity, as compared with air of 1·524, tends to occupy the deeper parts of excavations, and renders it unsafe, when they have been disused, to enter them without precaution. Sulphuretted hydrogen (HS) here and there evolved continuously, very poisonous, but readily detected by its offensive smell, is also slightly heavier than air; carbonic oxide (CO),

most deadly, but occurring rarely from natural causes, is 0.970. Fire-damp, or light carburetted hydrogen (CH), the grisou of the French miners, has a specific gravity of 0.555, and is therefore commonly found to float along the upper portion of levels, to escape of itself from workings carried downhill, and to lodge in hollows or the higher parts of excavations. If mingled with air in the proportion of th to th, it may be detected by the "cap" on the flame of a candle or lamp. If in larger proportions, it becomes explosive, and is most violent when it forms 4th or 4th of the mixture. The presence of carbonic acid greatly reduces the explosive property. When there is as much. as 4th of the gas, it burns without explosion, and a still larger proportion causes suffocation. In fiery seams it may be observed exuding from the freshly-broken surfaces with a hissing sound; and if in large quantity, as with "blowers," blowers," or sometimes near faults, with a rushing noise, like the steam from a high-pressure boiler. Under these last circumstances it will rise through a column many yards high of water, and numerous accidents have occurred through a forgetfulness of this property. Some of these blowers will be exhausted in a few minutes, others will last for years, -like that at Walisend, which gave off 120 feet of gas per minute and may be then piped off and burned at the pit bottom. The evolution of the gases from the coal is greatly affected by the pressure of the atmosphere, a notably larger amount being emitted when the barometer is low; and hence that instrument becomes a useful adjunct in judging of the amount of ventilation needed at different times.

For the due ventilation of a colliery, it is therefore

not sufficient to supply air enough for the breathing of men and horses and the burning of lights; but we must provide for the sweeping away of the products of breathing and combustion, for the removal of the gaseous results of blasting and of the decomposition of vegetable and animal matter; for the cooling of the excavations where the temperature is high partly from depth and partly from chemical change; and, lastly, for the dilution of the gases exuding from the coal.

In round numbers, 100 cubic feet of air per minute may be required for the health and comfort of each person underground, or for 100 men 10,000 cubic feet; but if fire-damp be given off-say at the rate of 200 cubic feet per minute-we should need at the very least thirty times that amount of fresh air to dilute it, or 6,000 cubic feet in addition. Increase the number of men and liability to gas, and 40,000 or 60,000 cubic feet of air may be indispensable for safety. Hence, we may point out once for all that no system of pipes can ventilate a mine, and that the large volumes of air required must be introduced through the drifts or workings themselves.

The subject now divides itself into two parts-first, the production of a current or "draught;" secondly, the distribution through the workings of the current so produced.

A spontaneous ventilation is produced by natural causes, which may always greatly assist, and, in some cases, may be sufficient for all purposes. To account for this on the simplest principles, let us observe what happens in summer and in winter with a diagrammatic working connecting two shafts of different depths.

The temperature of the rock is found, as we descend,