Besides the danger of being crushed to death by a fall of rock, or immured in a living tomb by an obstruction of the shaft or an irruption of water, the miner has another, and often still more formidable, enemy to encounter in the noxious gases frequently evolved in coal-pits. Thus in all well-regulated mines the greatest attention is paid to ventilation, so that no part of the workings may be left without a proper supply of air. In ordinary cases the natural currents, which set in different directions through the shafts and galleries, may sufficiently purify the atmosphere; but in the coal mines which are peculiarly subject to the evolution of foul gases, artificial or mechanical means must be resorted to for driving away the hurtful vapours as quickly as they form. To establish a proper air current, the usual method is to keep a large fire continually burning at the bottom of one of the two shafts of the pit, or of one of the two compartments of the single shaft, and the difference of temperature thus caused between the column of air of the _upcast_ shaft and the _downcast_ becomes the motive power which impels or drags the air current in obedience to it.[39] Yet this meets but half the difficulty; for the air current, which naturally tends to the shortest passage, must be forced to do its duty in every corner of the pit, and not suffered to escape through the upcast shaft before it has performed the longest circuit. For this purpose a great number of mechanical contrivances are adopted, in the shape of ‘stoppings,’ of brick, or wood, or stone, all so placed as to divert Footnote 39: Mr. Samuel Plimsoll (‘Letters on the Iron Trade,’ _Times_, February 10, 1868) informs us that in the Belgian coal mines the ventilation is carried on in a more economical and effective manner. Here no furnaces are lighted at the bottom of the _upcast_, because one-twentieth of the coal required for a furnace will make steam for an engine to work fans which act somewhat in the manner of huge paddle-wheels in steam- ships, and by rapid rotation over the shaft produce a draught which the incoming air rushes to meet, and thus powerfully promote ventilation. These fans they can work and control, and are therefore independent of those atmospheric influences to which some of our greatest calamities have been ascribed—the damp, heavy atmosphere of early winter. In the great colliery of Sacrée Madame, near Charleroi, one of these fans will draw 34,000 cubic metres (about 918,000 cubic feet) per minute.