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The Davy lamp does not emit a strong light; hence if it can beproved that the electric light will not set fire to the inflammablegases of the mines in the event of accidental breakage of theprotecting glass globes, its intense light should prove very valuableto the miner.
It has been found by experiment that the presence of coaldust in the workings contributes much to the risk of explosions;and it seems certain that if 3 per cent. of gas exists in the airof a mine which is thoroughly mixed with coal dust, an explosivemixture is formed. Mines should have at least two shafts, oneof which serves to admit the pure air, while the foul gases escapeby the other. The ascent of these gases is facilitated by creatinga draught by means of a furnace at the bottom of one of the shaftsor by fans driven at a high speed placed at top. This shaftis called the upcast shaft; the downcast shaft, which may be withina few yards of the other, allows the fresh air to pass down to theworkings, to the faces of which it is directed by partitions of woodor canvas called “brattices.” The air in its circuit below willtravel several miles.
The coal lies in parallel layers, between which the gas exists ina highly compressed state. In order to detach these layers withthe least possible danger, it is usual to cut through them endways,by which means the gas is allowed to make its escape at once froma considerable portion of the coal. From observation of somemines it is seen that discharge of fire-damp, though governed byatmospheric pressure, takes place before being indicated by thebarometer, so that, as an indicator, that instrument cannot be reliedon. As before said, the explosion of “fire-damp” in a mine resultsin an accumulation of the dangerous “ after-damp,” 1 and more livesare lost by it than by the explosion itself. It has the appearanceof a dense misty vapour, and resists the application of Ventilationin an extraordinary manner. It benumbs the facu.lties and deprivesthe miner of all presence of mind, so that, instead of rushing atonce to the pit bottom, if he has escaped the fire, he gets bewild-ered, and a deadly lethargy comes over him, ending in sleep fromwhich he never awakes. It is rare to find choke-damp and fire-damp in the same workings, or if they do occur it is only in smallquantities.
1 A mixture of carbonic acid and other products of the combustion of the carburettedhydrogen.