deal of coal left in one way or other, often about oneeighth of the original mass, occasionally so little as one-tenth; but all this depends upon such a variety of circumstances, that no positive proportion can be mentioned as true of a variety of localities.

In some of the Staffordshire collieries, where the coal is of such an amazing thickness, immense sections are excavated with a comparative impunity of danger; pillars of coal being left at intervals to support the roof, which from its height, and the vastness of the workings carried on beneath it, presents, when lighted up, a most impressive spectacle. The following is a ground plan of the old Bradley mine,

near Bilston. There are two shafts adjacent to each other A A. The horse gate is indicated by B B, and the narrower gallery, commencing at one of the pits and terminating in the open workings at c, is the air way. The stronger dark por

tion of the engraving is the rib, surrounding the work; D is a bolt hole or passage through the rib; and E E E are bolt holes stopped to cause a circulation of air through the work. F is a mass of coal left to support the shaft. The masses severally marked G, are the pillars which support the roof: the two smaller ones H H, are called by the miners "man of war" pillars. The excavated spaces in the direction

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217

of the darts are merely called openings: those in the cross direction, stalls.

In the method of proceeding by narrow or long work, as practised about Newcastle, the excavations, as already intimated, are conducted on a much more complicated plan than those just described. To give some idea of the mode, it may be stated that long passages are cut through the coal, generally in a straight line these openings are crossed by others at right angles, and 4 or 5 yards wide, the solid intervening masses forming rectangular pillars of coal 20 yards by 9, or, in some instances, square cubes of about the latter measurement on each side. It is in collieries of amazing depth, and containing many miles of excavation thus arranged, that perfect ventilation becomes an affair of equal difficulty and importance.

The efficiency of any system of ventilation conducted upon the ordinary principles, must depend mainly upon an attention to three particulars:-1, upon the opening of proper apertures for the admission of the atmospherical current; 2, on some method of accelerating, and by this means renewing, the volume of air in progress through the mine; and 3, in such a construction of the underground works, that every part shall be exposed to the ventilation. To illustrate this description-suppose the annexed diagram (fig. 28.) to represent a tube, having each end bent upwards at right angles with the horizontal portion, so as to form one level and two upright limbs. In this state, the tube will be filled with air

Fig. 28.

A

B

throughout, just as it might be filled with water,

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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219

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 н, 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 Fig. 30.