the furnace is considered to be less economical, and is forbidden by law in fire-damp collieries, and even associated with dumbdrifts is being abandoned, in order to be replaced by ventilators. (Mackworth's Evidence, June 23, 1853.) They are very generally applied to colliery ventilation.

Mr. John Taylor, of Tavistock, invented a form of air pump, which is thus decribed in Brewster's Edinburgh Encyclopdia :"A large cistern is constructed of wooden staves, hooped with iron, circular, and from 6 to 8 feet in depth; this is nearly filled with water. Through the bottom of this vessel a pipe passes from the mine to be ventilated, and passing up through the water, is carried about a foot above it. Upon the top of this pipe is an air-tight valve, opening upwards; over this pipe and within the sides of the cistern, a cylinder of plate iron is placed, open at the bottom but close at the top, in which top an air-tight valve is placed, also opening upwards. This iron cylinder is made to move in a vertical direction by guides or sliders, and its upper end is attached to a lever or chain, which is moved either by a water-wheel or steam-engine. An exhausting machine of this construction may be made from the smallest size to be worked by the hand, to any requisite to be moved by machinery (vol. 14, 1820).

According to a statement made by Mr. Wood, at a meeting of the North of England Institute of Mining Engineers, on June 3, 1853, a machine of Mr. Struve's invention, having an areometer, 17 feet in diameter, and worked at from 7 to 8 strokes per minute by a steam engine of 14 horse power, produced a ventilation of 22,000 cubic feet per minute, being 23ths of its calculated effect. In addition to the above, may be mentioned the centrifugal ventilation of Mr. Brunton, the pneumatic screw of M. Motte, the windmill ventilation of M. Lesoinne, the spiral ventilation of M. Pasquet, the inclined vane fan of M. Letoret, the curved vane fan of M. Combes, the feathering fan of M. Lemielle, and the pneumatic wheels of M. Faboy; the chief of which are minutely described and illustrated in Mr. Mackworth's evidence, contained in the First Report from the Select Committee of the House of Commons, on Accidents in Coal Mines in 1857.

The whole of these machines are liable, however, to one insurmountable objection: their liability to derangement, and the

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consequently unventilated state of the mine until they can be repaired. The heat of the upcast shaft of a mine, ventilated by furnace, is such as to cause a considerable circulation of air for many hours after it has been extinguished; and a proper arrangement with regard to dumb-drifts for currents which might chance to be dangerously loaded with explosive gas, will preclude the slightest risk arising from their passing into close proximity to the fire.

That air, put in motion, has momentum in common with other bodies, no one will doubt; but that its momentum in mines is almost instantaneously overcome by the friction or resistance of the air channels, is equally true. In proof of this may be stated a fact, elicited during the application of the exhausted steam from an underground steam engine to ventilate Belmont colliery, where each stroke of the engine was distinctly observable in the workings by its effect upon the doors; and in describing the ventilation by Mr. Struve's machine (before alluded to), Mr. Wood stated that the pulsation of each stroke of the areometer piston was distinctly felt in the workings at a considerable distance from the shaft.

THE OBJECTS TO BE PROPOSED IN THE VENTILATION OF MINES.

There are three principal objects which ought to be proposed in the ventilation of mines; namely, the introduction of a sufficient quantity of air; its proper distribution; and the security of the arrangements to maintain this distribution and circulation. In the first place, the quantity of air to be introduced should be not merely that which is necessary barely to neutralize and carry off the explosive and noxious gases which are yielded under ordinary circumstances in the mine; but also to supply a sufficient volume to provide for the unusual emissions of them, which sometimes occur, and to dilute these to such an extent as to avoid danger; and at all times to afford a healthy atmosphere for the breathing of the workmen. In the second place, due regard must be given to the proper distribution of the air introduced, so that every part of the excavations of the mine may have its requisite circulation, and none be left stagnant for the accumulation of gas; to the conducting of that which is introduced, without waste, and without its coming in contact with the dangerous and noxious gases, to those parts where the men are engaged; and to the providing for

its separate return or escape, or that of any portion of it, carrying off with it these gases, as soon as it shall have become anywhere perceptibly loaded with them, without again entering the working districts or coming in contact with the workmen or their lights. In the third place, the security and stability of the air ways, their freedom from leakage, and strength to resist the shock of an explosion, and also the regular and equable action of the motive power employed to produce the circulation, require attention.

In determining the requisite measures to effect these objects, it is necessary to consider the following subjects. The nature of the seam to be worked; its thickness and inclination; the character of the accompanying strata; the extent of the workings which are to be effected, and that of the surface of coal which will be exposed at one time in the excavations; the state of these works ings at different and future periods; and the number of workmen who will be employed.

The number of workmen to be employed is one of the most important elements in determining the quantities of air which ought to be introduced into a mine. The data which I have collected on this subject would induce me to assign an amount varying from 250 cubic feet per minute, in coal seams which are not fiery, up to 500 cubic feet per minute in fiery seams, as requisite for each person employed.

Attention cannot be too strongly called to the importance of measuring and recording, at stated times, the quantities of air passing through the different parts of mines; and that, not only in the ingoing and outcoming passages contiguous to the shafts, but also in those distant parts where the majority of the men are employed. It will be found that the volumes measured near the shafts often give erroneous results, if employed to estimate the state of ventilation in the more distant districts, on account of the leakage which occurs in the passage allowing a large portion of the air to return before it reaches the face of the workings. The investigation of numerous cases clearly shows that the greater part of explosions occur in collieries in which the defective supply of air, especially in the working districts, would become immediately evident, if submitted to measurement. The practice of ascertaining the exact quantities of air circulating in mines, espe cially if brought into comparison with the number of men em

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ployed, would also direct attention to the injury to health produced in numerous cases by insufficient ventilation.

THE MOTIVE POWER APPLICABLE TO COLUMNS OF AIR CIRCULATING
IN MINES.*

The following subjects require to be considered in the next place; namely, the motive power applicable to ventilation in mines; the velocities of air currents, particularly as connected with the loss of effect from friction; the relation of velocity and friction to the area, form, and length of air passages; and the separate volumes into which the total amount of ventilation should be divided.

THE VENTILATING FURNACE.

The effective power of furnace ventilation (which from its general use, where large quantities of air are needed, may be taken as the standard of reference) depends on the height and sectional area of the ascending and descending columns. In deep shafts the diminution of temperature, in the upper part of the ascending column, is considerable, and reduces the ratio of effect due to the height. In shallow shafts the want of height in the column is not found inconvenient, as the loss of effect, from this cause, may be compensated by giving a larger sectional area to the shafts; whilst at the same time, the length or run of the air currents is generally so much shorter, in shallow mines, and the quantities of air required are also so much less, that the apparent objections to its use under such conditions are not regarded.

some

The furnace fulfils the most important requisites in motive of power, by its efficiency, its uniform action, and the easy control of which it admits. The steam jet is inferior to it in these respects, but it is free from objections to which the furnace is open. The liability to derangement in all machines appears to preclude their use in ventilation, in other than exceptional In mines of large extent it would be difficult, perhaps impossible, to withdraw the workmen, in case of breakage, before danger ensued. The comparative effect, from a given consump tion of fuel, between the furnace and the most approved machines, is not favorable to the latter.

cases.

*Blackwell's Report.

The objections to the furnace are, the possibility of its exploding foul returns from the mine, either when this state is permanent, or produced by a sudden emission of fire-damp, or caused by the derangement in the general system which follows an explosion; and also the inconvenience and damage produced by an elevated temperature in shafts applied to winding. The first objection, relating to the danger from permanently foul returns, has been met by feeding the furnace by a split from the intake air, or by returning for this purpose a part of the air which has only had a short run and contains little inflammable gas, while that part of the return air which is explosive is passed by a dumb drift into the upcast shaft; or, otherwise, by such an increased ventilation in the mine as shall sufficiently dilute all the returns from every district, and permit the whole to be passed over the furThe latter of these plans is in general preferable, not only from the improvement in the state of the ventilation which it necessitates, but also from its allowing the whole of the returns to be rarefied, and thus enabling a larger effect to be obtained from the increased temperature. The second objection arises from the possibility of large emissions of fire-damp suddenly rendering the return air currents explosive. In fiery seams, which are newly opened, especially when the goaves begin to be formed, it may be advisable to employ the steam jet, the waterfall, or some other motive power than the furnace. The last objection may always be obviated by an additional shaft applied to ventilation alone.

nace.

In examining the effective power of the furnace, and comparing the results obtained in different instances from given temperatures, the drag, or comparative friction, in air courses of varying sizes, forms, and lengths, must be taken into account. A comparatively high temperature, in the upcast shaft, is necessary to obtain the largest economic effect, since the amount of difference between that of the upcast and downcast is, up to a certain point, expended in counteracting the resistance from friction consequent on setting the air in motion. After this point has been passed, all additional increase in the temperature would be utilized, or produce a corresponding effect, provided the area of the air passages were proportionally enlarged, as the temperature ascends, so as to maintain a constant or uniform velocity in the air currents. But if this rate of velocity be an ascending one, from the area of the air passages being constant, the resistance from this