temperature was reduced to 75°; and then the cost of the same work in the same gallery was £5 per fathom, instead of £17, as before; that is, a saving of £12 in excavating every six feet was realized by sending down enough of air.

The quantity of air breathed by a man in one minute is rather less than half a cubic foot, or exactly 320 cubic inches, according to chemists;* of which 10 per cent., consisting of oxygen, is consumed by the lungs, and from 77 to 8.5 parts of carbonic acid gas are expired. A candle burns every minute about one seventh of a cubic foot of air, and it will begin to burn dimly when the proportion of oxygen in the air is diminished from 21 per cent. to 18 per cent.; while it will be extinguished when the proportion falls to 16 per cent., and at the point of 14 per cent. suffocation will gradually ensue. From these ascertained data, it is not difficult to estimate what is the state of a mine where men have worked for some five or six hours, and have burnt numerous candles; but we have also data in the analysis of 18 samples of air taken in four Cornish mines, at an average depth of 214 fathoms (1284 feet), and an average distance of 28 fathoms (168 feet) from any shaft. The result of this analysis affords a mean per-centage of 17.067 oxygen, 82-848 nitrogen, and 0·085 carbonic acid gas. The air had been deprived of much oxygen by the breathing of the men, the combustion of the lights, and the decomposition of mineral and animal substances.

Besides the vitiation of the air by the presence of a large body of miners, we have this made sensible in another form, by the raising of the temperature. Fifty miners, with their lights, will give off sufficient heat to raise fifty thousand cubic feet of air, being at a temperature of 55°, by one degree every minute. Now there are in most mines of any magnitude many more than fifty persons at work in them at the same hour. Towards the end of their work, therefore, the mine must be very oppressive; very far more oppressive than any room where a large party has been assembled for several hours, and has exhausted the oxygen of its limited atmosphere. What a crowded church or apartment is to a long inmate of either, that a badly aired coal pit is to the miners, with the addition of their being required to labour actively and exhaustingly in the midst of air more thoroughly vitiated than that of any building, and often corrupted with the efflux of noxious gases from the coal. It is not to be wondered at, that they work in the recesses of the mine half naked, and sometimes more than half; and that in some Cornish mines of great depth, where the

* According to Dr. Glover, 666 cubic feet of air will sustain a healthy man for 24 hours. Supposing a working place to be 12 feet wide, 4 feet high, and 4 feet long, the air in that place will sustain, with discomfort, a person for 7 hours.

Natural Ventilation.

47

temperature has approached 100°, owing to deficient ventilation, the men have been compelled to plunge into cold water several times during their short relays of painful work.

The ventilation of a coal pit is founded on well known natural laws, and is simple enough in the merely theoretical view. If there be two shafts sunk to the bottom, at a suitable distance from each other, one is called and used as the "downcast shaft," and the other as the "upcast shaft,"-the air descending by the former, and ascending by the latter. In a general sense the force of the ventilation depends upon the difference in weight between two columns of air, extending from the limits of the atmosphere to the bottom of two shafts, when they are on the same level. When one shaft is deeper than the other, allowance must be made for the difference between the weight of the air in it, and the weight of the air descending or ascending through the workings. A natural ventilation will always establish itself under such arrangements, just as in a room with two chimneys: when air has been made to descend one chimney and to ascend the other as warm air, then this warm air, having once filled the second chimney after passing through the room, establishes a rarefied column in the second, lighter than the unrarefied column in the first chimney. Let us assume that thirteen cubic feet of air weigh one pound, and that air expands or contracts one five-hundred-and-nineteenth part of its bulk at 60°, for every degree of increase or decrease in its temperature; then the actual weight of the air in each chimney or shaft can be accurately estimated. One column of air will balance the other, like the weights hung over a pulley; and the lighter column will ascend with a velocity which obeys the same mechanical laws as the pulley, and which can be equally well determined.

The science of natural ventilation, then, consists in so arranging, by the use of downcast and upcast shafts, the currents of air in a mine, that the cold air from the surface shall, by its density and dryness, descend at once to the lowest depths of the mine, and to its inmost recesses; and, passing along all the workings without leakage, shall supply all the workpeople in the mine with an adequate quantity of air for respiration, and also for absorbing and carrying away the heat arising from the mine, the men, and their lights, as well as the gases exuding from the coal, the moisture, and the various exhalations ;-so that they shall all unite in rarefying the air during its descent to the lowest part of the upcast shaft to which it can be conveniently conducted. Natural ventilation, however, would not suffice for large mines with large numbers of inhabitants: therefore little is known to miners of its principles; and artificial ventilation, or assistance derived from artificial aids, is in ordinary use. This we shall explain as succinctly as possible.

There are four principal kinds of artificial ventilating power, -the furnace, the steam jet, mechanical fans, &c., and water ventilators. The furnace acts by rarefaction,* and assists the natural tendencies of the common air. The steam jet acts partly by a similar rarefaction, and partly by its vigorous propulsion, which assists in exhausting the air. Mechanical ventilators are of various kinds, such as pumps, fans, screws, and pneumatic wheels, and of different economical values, according to the circumstances in which they are applied; some being good with a high velocity and a small "drag," others with a small ventilation and heavy drag, whilst the various inventions of this kind are severally distinguished by small first cost and heavy maintenance, or large first outlay with economy Water ventilators, unlike the other mechanisms, are more frequently used to force air into the mine than for exhausting air from it; and they are chiefly applicable to metallic mines, such as the Cornish. With the evidence published by the last Committee, there are printed engravings of machinery for colliery ventilation in Belgium, taken, as we think, from a Belgian work on the subject. A high authority remarks to us, in a private letter, that they are given without discrimination of their value, and that the only one worthy of attention is Fabry's; but the last form of this is not given. The same authority does not think it equal as a mechanical ventilator to Struve's aërometers, or air-pumps.

in use.

A furnace is the most convenient ventilating power, as long as the resistance of the air-ways in shallow mines does not amount to more than four pounds per square foot, and in deep mines to more than eight pounds per square foot. When the resistance is greater, mechanical ventilation may be employed. But the velocity of air-currents in mines is restricted, by the considerations of the convenience of men in using lights, the great leakage that takes place at high velocities, and other causes, to a speed commonly not exceeding from three to five lineal feet per second; and this speed can generally be attained with the furnace by judicious management, such as the enlargement and division of the air-ways. The viewers of the North are firmly wedded to the furnace system, "perhaps," acknowledges Mr. Nicholas Wood, the chief Northern viewer, " as an old friend, though we have had as a substitution innumerable plans and suggestions, few of which have stood the test of time and experience." The Parliamentary Committee announce that "the preponderance of evidence is decidedly in its favour." Those, however, who consider the subject apart from such influence

* It is estimated by some that the average limit of the furnace is the production of one thousand cubic feet of air per minute per foot arca. This is the standard rule of some districts. It is clear that the air cools as it expands, and at length will acquire a density similar to that of the air it encounters.

Ventilation by Furnaces.

49

and evidence, find some disadvantages in the furnace. Its ventilating effect is very irregular. The increase of the temperature of the atmosphere, between morning and mid-day, is sometimes sufficient to diminish the ventilation by one sixth. An unskilful flueman, or a choked fire, will lessen the effect by one tenth. An ordinary pit furnace is in size from eight to ten feet, and the length may be six feet. At South Hetton there is a furnace fifteen feet in width. The space under the bars should be left open, and the ash-pit kept full of water, as the heat which radiates downwards is thereby utilized. The amount of the ventilating current produced by two such furnaces was, in one mine, (Seaton Delaval,) fifty-three thousand cubic feet of air per minute,-the highest amount which the viewer, Mr. Forster, could obtain. Mr. Wood has performed a series of highly interesting experiments with the furnace and the steam jet at Hetton pits, from which we learn that the amount of ventilation obtained by a furnace nine feet wide at the Eppleton Jane pit, having an area of 58 feet, was, on an average, 49,296 cubic feet per minute. In the Minor pit, having 98 feet area, the average was 107,300 cubic feet of air per minute. The consumption of coals producing these effects, was at the rate of 10-11lbs. per minute; and 16,320 cubic feet of air were gained per minute from each pound of coal consumed. The workings of the Hetton colliery-which comprise one upcast shaft of 900 feet depth, and two downcast shafts respectively of 900 feet and 1,080 feet (the latter of which the writer of this article descended)-are ventilated by the three shafts just named, and with a power obtained from three furnaces, one nine feet in width, and the other two eight feet each. The whole workings are very extensive, and are carried out in three seams of coal; namely, the Hutton seam at 900 feet, the Low Main coal at 780 feet, and the Main coal at 660 feet from the surface. In the Hutton seam the workings extend over 2,000 acres, comprising coal partially worked or standing in pillars, and also coal entirely worked away, or goaf."

66

The furnaces burn 900 feet below ground in the Hutton seam; and though these furnaces embrace the ventilation of the entire workings in all the seams, the air from the Low Main and Main coal seams does not pass over them, but passes into the upcast shaft at their respective depths from the surface. The quantity of air passing round the workings of the Low Main coal seam is no less than 29,200 cubic feet per minute, and in the Main coal seam 29,950 cubic feet per minute. These are high quantities, and are only obtained in such complete and well appointed concerns as the chief Northern collieries. But we think the minimum amount of air, even for pits not fiery, should be from 10,000 to 15,000 cubic feet per

minute; nor should the split currents in fiery mines be less than from 8,000 to 10,000 cubic feet per minute.

A large amount of time and attention has recently been directed to the furnace, as the presumed best and most effective ventilating power, in comparison with what is termed "the steam jet," which Mr. Goldsworthy Gurney claims as his invention, or rather application. The Committee of the House of Commons which sat in 1851, only receiving evidence for five days, and publishing a Report of 247 pages, gave much prominence to the steam jet, and pronounced decisively on the matter, by saying, "Your Committee are unanimously of opinion, that the steam jet is the most powerful, and at the same time least expensive, method for the ventilation of mines." This appeared to the Northern viewers to be a startling declaration; and they regarded the whole as partaking of the character of special pleading, founded on very insufficient evidence. It has been pointed out that most of the questions and answers wore an ex-parte appearance; but the best result was, that it prompted Mr. Wood and others to institute a series of careful experiments in Northern pits, on the relative merits of furnace and steam jet. The results lie before us; and, as the consequence of a study of the whole, we are bound to infer that the merits of the steam jet have been exaggerated. The First Appendix to the Third Report of the Committee of 1853, is a full and complete exposition of the subject, and does great credit to its compiler, Mr. Nicholas Wood. The question must now be viewed as in a great measure determined. It is difficult to conceive of more detailed and patient experiments; and the issue of all, in brief, is as follows:

With respect to the application of the jet as a substitution for the furnace, and considered with reference to their comparative powers in producing the largest amount of ventilation; and, consequently, as having a tendency in that respect to prevent accidents in coal mines; these experiments show, in the most conclusive manner, that the steam jet, as hitherto employed, is far inferior to the furnace in producing a large amount of ventilation in deep mines; and the experiments at a shallow mine show that with equal areas of fire-grate the furnace is even superior to the steam jet in shallow mines.

There can only be one other question; namely, What is the value of the steam jet as an auxiliary to the furnace? In connexion with this question, numerous and accurate experiments were performed; and the issue is, that even as an auxiliary to the furnace, the steam jet is deficient. The increase of effect of the jet over the furnace is quite inconsiderable. Such increase is extremely unsteady, and in some cases nothing at all, when the furnace is urged to its maximum effect; and, in the ordinary working state of the furnace, amounts to only about