which would stand at the same height in each limb; of course, there will be no circulation*; so that should any circumstance occur to deteriorate the air in any part, especially in the horizontal portion, it would remain either stagnant near the spot where it was generated, or merely diffuse itself through the adjacent mass. And although the cause of deterioration might be of such a nature, that, by rarifying the internal volume in its vicinity, there would be a slight descent of fresh air, still, as the atmosphere would continue to press equally upon both apertures, nothing like circulation would take place, while the impure mass might be increased and concentrated. But if to the tube A we apply the flame of a lamp, or in any other way raise the temperature withinside, a new state of things instantly commences: a current of fresh air will rush down the tube B, and passing along the horizontal part c, will re-issue at A, thus ventilating, or, as it were, sweeping the entire passage and this operation will go on so long as heat is applied at A. This is, in fact, nearly the entire theory of simple ventilation; B representing the downcast pit or shaft, by which the air descends into c, or working drift; after sweeping which, it rises to mix with the atmosphere, by the upcast shaft A. And

* In practice, however, the result is somewhat different, owing to the temperature of one part of the mine being raised considerably higher than the other, owing to the presence of the workmen and other causes. The instant a dip-pit is connected with a rise-pit, by a drift, a strong circulation of air like wind commences. If the air at the surface is at the freezing point, it descends the dip or deepest pit, freezes all the water upon the sides of the pit, and even forms icicles upon the roof of the coal; but the same air, in its passage along the boardways to the rise-pit, which is generally of less depth, has its temperature greatly increased, and issues from the pit mouth in the form of a dense misty cloud, formed by the condensation of the natural vapour of the mine in the freezing atmosphere. Mr. Bald, in Edin. Phil. Journ. vol. I. 134, where a series of investigations on the temperature of air and water in coal mines may be found.

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if a colliery consisted simply, as is here supposed, of a single passage, however long or tortuous that passage might be, between one pit and another, no mode of ventilation could be better than this.

In practice, the following is the method of ventilation ordinarily adopted :-A (fig. 29.) is the down

cast shaft, and B the upcast; at a short distance from the bottom of the latter, and in the connecting passage c, is placed a furnace D, consisting of a platform of iron bars, raised somewhat from the ground, and covered with a fire of 7 or 8 feet in width, by 12 feet in length. The smoke and draught of this fire, instead of being connected with the bottom of the pit B, are provided for by the carrying of an arched drift E, in an inclined direction, from above the fire-place into the shaft, at a little distance from the bottom.* A stopping of boards is placed at F, to prevent access of the air in that direction. In what is called compound ventilation, a passage is carried from G, where there is also a furnace, to another downcast pit H, in an opposite direction to the first. Wallsend, Percy Main, Hebburn, and Heaton collieries, were all ventilated upon this principle.

* In ordinary cases, the smoke from the underground furnace, or other works, is conducted by a lateral flue near the top of the shaft into a capacious chimney (see fig. 26. p. 204).

So far the apparent object is merely the ventilation of a single passage, or open space, between the pits; but as, by a law of pneumatics, the air has a tendency to rush from the downcast to the upcast shaft by the nearest openings, a large portion of every tract of workings carried forward by the narrow plan as above mentioned, would be unvisited by the fresh current, if it were not compelled by certain contrivances to traverse all the galleries. To shew how this is effected, fig. 30. represents a plan of a small

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working ventilated by the compound method above alluded to, only that in this instance, instead of two downcast pits and one upcast, we have the three formed by bratticing or boarding a single shaft into three divisions, a e i-the two former letters indicating the downcast, and the latter the upcast, sections of the pit near to the bottom of the latter are the two furnaces. The darts shew the direction of the two currents of air from their respective downcast pits, to the bottom of the upcast pit. The single lines in the figure shew the headway brattices; the double lines, the stoppings; m m m m, the winning headways, driven in the longitudinal direction of the coal, or in the line of its bearing; n n nn, the stentings, or opening between the double headways; ww, the walls, or passages between the boards at the end of each pillar; bb bb bb, the boards or passages driven in the transverse direction of the coal, and at right angles out of the headways; the boards are the places from which the daily supplies of coal are obtained. Just beyond the darts on each side of x are the two furnaces; k k k are three arches by which the current of air is carried over its former course.

In the old method of simple ventilation, the air, after descending the downcast pit, would, as before stated, pass in the shortest line to the upcast, but was prevented by doors and stoppings, and carried round by the extremities of the workings to the upcast shaft, without ever visiting the interior parts of the mine, in which, consequently, the air was left to stagnate. If, therefore, a discharge of inflammable gas happened to occur in the old workings, as it not uncommonly did, it soon diffused itself, and, along with the stagnant air, got into the course of the at

mospheric current, rendering it nearly impossible for the workmen to prevent its exploding at their lights as they pass and repass along the working headways.

The inadequacy of this system, according to the testimony of Mr. Buddle*, was fully admitted; but no improvement was made, until the late ingenious Mr. Spedding, of Whitehaven, to whose memory our tribute of gratitude is due, suggested the idea of coursing the air; that is, forcing it through every passage of the several workings, or, in other words, making an air-pipe of every passage of the several workings of a coal mine: this sweeping of every part is effected by a complicated arrangement of stoppings and trap-doors; and so long as these are kept in order, and the passages open, it is almost impossible for there to be any accumulations of inflammable gas, owing to stagnation of the air in any part of the workings.

Besides the method of rarifying a volume of air in the upcast shaft, so as to promote ventilation by means of a furnace placed near the bottom, or as is sometimes the case, on the top of the pit, various other contrivances have been adopted, such as rarifying the air, by passing it through a large iron cylinder heated to a high temperature-the inflammable gases never igniting from contact with hot metal through which they may pass, though they would inevitably explode were they to come over an open fire. Exhausting and forcing pumps, worked by the engine, have sometimes been employed: Mr. Buddle prefers the former, because by the exhausting

* From whose interesting letter, addressed in 1813 to Sir R. Milbanke, Bart., on the various modes employed in the ventilation of collieries, these descriptions are derived.