350 Mr. Buddle's Narrative of the Explosion
through the workings, as to be explosive. This objection to the furnace is, however, greatly obviated by what I call the system of double ventilation. The principle of this system is to supply the furnace with a current of air, which is not exposed to the hazard of being contaminated by an admixture of inflammable air. While the current of air, which is exposed to such contamination, is delivered into the up-cast shaft by a separate out-let, so as not to expose it to the risk of coming in contact with the flame of the furnace. The annexed diagram explains the mode in which this is managed.
a the upcast shaft, b the furnace, c the drift, by which the contaminated air is carried off into the upcast shaft.
The furnace b, always heats the air in the shaft a, sufficiently to draw the current through the drift c. A furnace may be placed at d to be used occasionally, as it frequently happens in practice that the furnaces have to be worked alternately. The furnace d will draw the current of air as well through the b furnace drift, as the furnace b, will draw the current through the d furnace drift. Which ever furnace happens not to be at work is called the dumb furnace, and the drift leading from it into the shaft is called the dumb furnace drift. This system admits of great modification, according to circumstances, and the Bensham Seam, at Wall's End Colliery, has, from its first opening in 1820, been ventilated according to its principles.
The enormous discharge of gas from this seam rendered it necessary to have several distinct currents of air passing through the workings, some of which supplied the burning furnaces, and the remainder were carried off by
