LAMP OF DAVY consists of a com mon oil lamp, surmounted with a covered cylinder of wire gauze, for transmitting light to the miner without endangering the kindling of the atmosphere of fire damp which may surround him ; because carbureted hydrogen, in passing through the meshes of the cylindric cover, gets cooled by the conducting power of the metallic gauze, below the point of its ascension.
The frequency of explosions in coal mines in England led the Mining Board of that country to apply to Sir II. Davy in the hope lie might be able to apply some means for its prevention. The ex periments which he undertook for this end were numerous, and ended in the formation of this lamp. He found that the fire damp of the coal mine (carburet ted hydrogen) was not explosive unless mixed with air, and that ten times its volume of air was the most explosive proportions ; that even in these propor tions it did not explode unless a body in fall is, in Awns, were "hronalit in contact with it. lie also found that when the gas was inflamed and made to pass through metal tubes of small diameter, it cooled so much as not to set the surrounding gas on fire, and that it kept the inflamed and uninflamed portions perfectly distinct. On further experiment, he found that sections of tribes answered as well as tubes of any length, and hence a sheet of wire-gauze was equally efficient : he formed his safe tylamp therefore of a tube ofgauze, which prevented the inner flame of gas from spreading outwards into the great body of fire damp. It has been the means of
saving many hundred lives ; but miners will carelessly open it to have more light to work with, or to light their pipes, when if the fire-damp be in the passages an explosion must occur. Improvements have been made in Davy's lamp by Drs. Reid and Clanny, by substituting for the lateral wire gauze a glass shade closed at top with gauze.
The apertures in the gauze should not be more than 1-20th of an inch square. Since the fire-damp is not inflamed by ignited wire, the thickness of the wire is not of importance, but wire from 1-40th to 1-60th of an inch in diameter is the most convenient.
The cage or cylinder should be made by double joinings, the gauze being fold ed over in such a manner as to leave no apertures. When it is cylindrical, it should not be more than two inches in diameter ; because in larger cylinders, the combustion of the fire-damp renders the top inconveniently hot; a double to is always a proper precaution, fixed half or three quarters of an inch above the first top.
The gauze cylinder should be fastened to the lamp by a screw of four or five turns, and fitted to the screw by a tight ring. All .ioinings in the lamp should be made with hard solder, as the secur ity depends upon the circumstance that no aperture exists in the apparatus larger than in the wire gauze.