F, the wire gauze cylinder, which should not have less than 625 apertures to the square inch. G, the second top, three-fourths of an inch above the first, surmounted by a brass or copper plate, to which the rings of suspension are fixed. I, I, I, six thick vertical wires, joining the cistern below to the top plate, and serving as protecting and strengthening pillars round the cage. When the wire-gauze safe lamp is lighted and introduced into an atmosphere gradually mixed with fire-damp, the first effect of the fire-damp is to increase the length and size of the flame. When the inflammable gas forms as much as 1-12th of the volume of the air, the cylinder becomes filled with a feeble blue flame, but the flame of the wick appears burning brightly within the blue flame, and the light of the lamp augments, till the fire-damp increases to one-eighth or one-fourth, when it is lost in the flame of the fire-damp; which in this case fills the cylinder with a pretty strong light. As long as any explosive mixture of gas exists in contact with the lamp, so long it will give light; and when it is extinguished, which happens when the foul air constitutes as much as one-third of the volume of the atmosphere, the air is no longer proper for respiration; for though animal life will continue where flame is extinguished, yet it is always with suffering.
By fixing a coil of platinum wire above the wick, ignition will continue in the metal when the lamp is itself extinguished; and from the ignited wire, the wick may be again rekindled in going into a less inflammable atmosphere. In a letter to the Royal Society, dated Newcastle-upon-Tyne, Sir H. Davy says, "All the lamps that I have examined, have at different times been red-hot, and a workman at the Hepburn Colliery showed me a lamp, which, though it had been in use about sixteen hours a-day for nearly three months, was still in excellent condition; he also said it had been red-hot, sometimes for several hours together. Wherever workmen, however, are exposed to such highly explosive mixtures, double gauze lamps should be used; or a lamp in which the circulation of the air is diminished by a tin plate reflector, placed in the inside; or a cylinder of glass, reaching as high as the double wire, with an aperture in the inside; or slips of Muscovy glass may be placed within this lamp; and in this way the quantity of fire-damp consumed, and consequently of heat produced, may be diminished to any extent. Such lamps, likewise, may be more easily cleaned than the simple wire gauze lamps; for the smoke may be wiped off in an instant from the tin plate or glass.
If a blower or strong current of fire-damp is to be approached, double gauze lamps, or lamps in which the circulation of the air is interrupted by slips of metal or glass should be used, or if the single lamp be employed, it should be put into a common horn or glass lantern, the door of which may be removed or open."
Notwithstanding the increased security afforded by the safety lamp, coal miners are slow to avail themselves of it, owing to the inferior degree of light it affords compared with that given by a naked candle. This arises from two causes, viz. the necessary obstruction offered by the black wire, of which the cage or gauze is composed, within which the lamp is placed; and the casual obstruction occasioned by the adhesion of smoke to the inside of the cage, when the lamp is not carefully trimmed, and of smut and dust to the outside of the cage.
To obviate these objections, Mr. Roberts, of St. Helen's, Lancashire, has introduced some modifications and improvements in the construction of the safety lamp, for which he has received a reward from the Society of Arts. To diminish the obscuration occasioned by the first cause, Mr. Roberts proposes that the wire shall be kept bright and polished, by cleaning the cage every night with a soft brush, and the black powder, or smut, which occurs in all coal mines, especially in the neighbourhood of faults; this smut is pulverulent non-bitumenous coal, sufficiently hard to remove the rust from the surface of the wire, without materially wearing the wire itself. As the lamp is at present constructed, the oil will run out of the cup or receptacle in which it is placed, if the lamp is laid in a horizontal position, an accident which frequently occurs on account of the lamp being rather top heavy. When this happens, the gauze becomes smeared over with viscid oil, which causes the coal dust floating in the air of the mine to adhere to it, and in a short time to fill up, more or less, the meshes of the gauze.
By merely shaking or tapping the lamp, the dust will not be dislodged; and if the miner attempts to clear his lamp by blowing through the wire gauze, he runs the risk of putting out the light, and, after all, very imperfectly clears the meshes; there is also, perhaps, some risk of forcing the flame through the meshes on the opposite side, and of producing an explosion, if the surrounding air is inflammable. In Mr. Roberts's lamp the overflow of the oil is impossible, on account of the dome-shaped cover which surrounds the wick; the dust, therefore, that settles on the gauze may be dislodged by a mere tap of the finger, or what would perhaps be better, by the application of a small brush similar to that which soldiers carry to clear the pan of their muskets, and which might be attached by a bit of small chain to the handle of the lamp. Fig. 1, on the preceding page, represents a section of the lamp pp, and wire gauze q q; rr, a screwed cap, with a hollow dome s; it screws into the neck, t t, of the lamp; the dome rises a little above the neck holder u, having an opening at top to let the wick and trimming wire v, rise through.
This dome serves to catch and retain any oil that may be spilt by shaking the lamp, or knocking it over, thereby protecting the wire gauze q from being smeared: w and x, two locks, the former to secure the cap q, and the latter to secure the wire gauze q from being removed. Fig. 2, a section of the cap and dome, rrs, separate from the lamp; the wire gauze fits into the cavity y y, around the dome s; z z, two of the four wires which serve to hold the wire gauze.
Mr. Bonner, of Monkwearmouth, Durham, has a patent for an improvement upon the safety lamp, which consists in a means of increasing the light of the lamp, and also of extinguishing it instantaneously. The mode of increasing the light is as follows: - Instead of introducing a wick in the centre of the lamp, as is usually practised, he introduces a series of small wicks round a centre tube, and by lighting one, two, or more wicks at a time, little or much light is obtained. The means of instantly extinguishing the light consists in a metal cap, or extinguisher, suspended within the wire gauze tube by a pin or catch; upon withdrawing the pin the extinguisher falls over the wick and the light is put out.
In Mr. Murray's safety lamp the wire gauze tube is suspended by two concentric tubes of strong glass, the space between the two tubes being nearly filled with water; by this means a much greater degree of light is obtained, but we are not sure that the risk is not also greater than when a wire gauze tube is employed.