There is another modification of the ventilating system, where the air-courses are traversed across; that is, when one air-course is advanced at right angles to another, and must pass it in order to ventilate the workings on the further side. This is accomplished on the plan shown in [fig. 863.], where a is a main road with an air-course, over which the other air-course b, has to pass. The sides of this air channel are built of bricks arched over so as to be air-tight, and a gallery is driven in the roof strata as shown in the figure. If an air-course, as a, be laid over with planks made air-tight, crossing and recrossing may be effected with facility. The general velocity of the air in these ventilating channels is from 3 to 4 feet per second, or about 21⁄2 miles per hour, and their internal dimensions vary from 5 to 6 feet square, affording an area of from 25 to 36 square feet.
Mr. Taylor’s hydraulic air-pump, formerly described, [p. 839.], deserves to be noticed among the various ingenious contrivances for ventilating mines, particularly when they are of moderate extent. a is a large wooden tub, nearly filled with water, through whose bottom the ventilating pipe b passes down into the recesses of the mine. Upon the top of b, there is a valve e, opening upwards. Over b, the gasometer vessel is inverted in a, having a valve also opening outwards at d. When this vessel is depressed by any moving force, the air contained within it is expelled through d; and when it is raised, it diminishes the atmospherical pressure in the pipe b, and thus draws air out of the mine into the gasometer; which cannot return on account of the valve at e, but is thrown out into the atmosphere through d at the next descent.
The general plan of distributing the air, in all cases, is to send the first of the current that descends in the downcast shaft among the horses in the stables, next among the workmen in the foreheads, after which the air, loaded with whatever mixtures it may have received, is made to traverse the old wastes. It then passes through the furnace with all the inflammable gas it has collected, ascends the upcast shaft, and is dispersed into the atmosphere. This system, styled coursing the air, was invented by Mr. Spedding of Cumberland. According to the quantity of the fire-damp, the coursing is conducted either up one room, and returned by the next alternately, through the whole extent of the works, or it passes along 2 or 3 connected rooms, and returns by the same number.
This admirable system has received the greatest improvements from the mining engineers of the Newcastle district, and especially from Mr. Buddle of Wallsend. His plan being a most complete scale of ventilation, where the aerial current is made to sweep every corner of the workings, is shown in [fig. 865.]; in which a represents the downcast, and b the upcast shaft. By pursuing the track of the arrows, we may observe that the air passes first along the two rooms c, d, having free access to each through the walls, but is hindered from entering into the adjoining rooms by the stoppings which form the air-courses. It sweeps along the wall faces of the rooms c, d, and makes a return down the rooms e, f, but is not allowed to proceed further in that direction by the stoppings g, h. It then proceeds to the foreheads i, k, and single courses all the rooms to the foreheads l, m; from this point it would go directly to the upcast pit b, were it not prevented by the stopping n, which throws it again into double coursing the rooms, till it arrives at o, whence it goes directly to the furnace, and ascends the shaft b. The lines across each other represent the passing doors; and these may be substituted in any place for a passage where there is a stopping. The stopping p, near the bottom of the downcast shaft, is termed a main stopping; because if it were removed, the whole circulation would instantly cease, and the air, instead of traversing in the direction of the arrows, would go directly from the downcast pit a, to the upcast pit b, along the gallery q. Hence every gallery and room of the workings would be laid dead, as it is termed, and be immediately filled with fire-damp, which might take fire either at the workmen’s candles, or at the furnace next the upcast shaft b. Thus also a partial stagnation in one district of the colliery, would be produced by any of the common stoppings being accidentally removed or destroyed, since the air would thereby always pursue the nearest route to the upcast pit. Main stoppings are made particularly secure, by strong additional stone buildings, and they are set up at different places, to maintain the main air courses entire in the event of an explosion; by which precautions great security is given to human life. This system of ventilation may be extended to almost any distance from the pit-bottom, provided the volume of fresh air introduced be adequate to dilute sufficiently the fire-damp, so that the mixture shall not reach the explosive point. The air, by this management, ventilates first one panel of work, and then other panels in succession, passing onwards through the barriers or panel walls, by means of galleries, as in [fig. 843.], by the principle either of single, double, or triple coursing, according to the quantity of gas in the mine.
In ventilating the very thick coal of Staffordshire, though there is much inflammable air, less care is needed than in the north of England collieries, as the workings are very roomy, and the air courses of comparatively small extent. The air is conducted down one shaft, carried along the main roads, and distributed into the sides of work, as shown in [fig. 848.] A narrow gallery, termed the air-head, is carried in the upper part of the coal, in the rib walls, along one or more of the sides. In the example here figured, it is carried all round, and the air enters at the bolt-hole e. Lateral openings, named spouts, are led from the air-head gallery into the side of work; and the circulating stream mixed with the gas in the workings, enters by these spouts, as represented by the arrows, and returns by the air-head at g, to the upcast pit.
When the fire-damp comes off suddenly in any case, rendering the air foul and explosive at the foreheads, if no other remedy be found effectual, the working of the coal must be suspended, and a current of air sent directly from the fresh in-going stream, in order to dilute the explosive mixture, before it reaches the furnace. This is termed skailing the air; for otherwise the gas would kindle at the furnace, and flame backwards, like a train of gunpowder, through all the windings of the work, carrying devastation and death in its track. By skailing the air, however, time is given for running forward with water, and drowning the furnace. A cascade of water from the steam engine pumps is then allowed to fall down the pit, the power of which through a fall of 500 or 600 feet, is so great in carrying down a body of air, that it impels a sufficient current through every part of the workings. The ventilation is afterwards put into its usual train at leisure.