In the Stephenson lamp the air is admitted through only a few meshes of the wire gauze within a glass cylinder, the latter preventing the entry of any air or gas from the sides: consequently, but a small portion of gas is allowed to enter, and, the interior of the lamp never being filled with flame, no injury can result. In explosive mixtures there not being a sufficiency of oxygen to support combustion, and not a sufficient quantity of gas to support the requisite temperature for its inflammation, the light is extinguished.
The Stephenson lamp is, therefore, a peKfietly safe one, and not liable to the objections offered against the use of these lamps in the preceding pages, because it will not burn in a dangerous or explosive element.
The Clanny lamp, as improved, produces more light than either the Davy or the Stephenson, but is more dangerous than the latter, and perhaps than the former, be cause it is protected on the sides by glass only. The principles of insulating the flame and passing the air in and out of the lamp through wire gauze are the same as in the lamps mentioned: in fact, all the improved safety-lamps—and there are a great many of them—contain these principles of the Davy lamp.
Among others, we may mention, in addition to the foregoing, the Museler, a Belgian lamp, much like the Clanny ; the Boty, a modification of the same; and the Eloin, with an argand or flat burner, a glass cylinder, and a copper cap, something like those used in our coal-oil lamps, around the flame. It is easily extinguished. In addition to these are the Glover lamp, with a double glass cylinder for protection in case of accident, the Upton & Roberts lamp, which "goes out" in an inflammable mixture, and the Hall and Fife lamps, having a double cap of wire gauze, with glass cylinders.
When lamps on the principle of the Davy, with a covering of wire gauze alone, are used, great precaution should be used in withdrawing them when the internal combus tion of the gases takes place, or when the wires of the gauze become red-hot, as an explosion is then imminent.
Lamps having glass cylinders should be carefully used; and the admission of air into the interior of the lamp should be so regulated as to prevent the combustion of the gases within the cylinder ; while those with single tops of thin wire gauze are not a suffi cient protection against accident.
The Stephenson lamp and those on the same principle are the only true safety-lamps.
They will not burn in a dangerous or explosive mixture, and, consequently, cannot be used where men should not be allowed to work. The great objection to the " Geordie" is its small emission of light. The same principle, however, has been improved by Mr. T. Y. Hall, and others, until their lamps emit more light than the Clanny or the Davy, and is so constructed that they will not burn in a dangerous mixture, and yet continue to give light until the danger is really imminent. Precautions against accidents are taken; and the miners cannot light their pipes by sucking the flame through the sides, as in the Davy and other lamps.
Under such improvements, a real safety-lamp may be had, against the use of which there can be no objection, except those before made against the general use of safety lamps in mining operations as a protection from the danger which ought rather to be removed. This can be done by the ordinary means of ventilation ; but, as we have shown, this dangerous and troublesome element may not only be rendered inoffensive and safe, but may be converted into an active agent in promoting perfect ventilation. The plan is simple, economical, and entirely available. We commend it to the attention of all those who may have fire-damp to contend with ; and we may restate the principle in a few words.
Carburetted hydrogen gas, or the fire-damp of our coal-mines, being very light, and having a much greater ascensional power than the air rarefied by our ventilating furnaces, always ascends to the highest points of the mine or working, seeking the means of escape; and if those means were provided the gas would rush out of the mine with great velocity. Therefore, if the return air-courses were so constructed that the gas would invariably ascend to them and in no case have to descend in its exit, it would not only escape as fast as it issues from the fissures of the coal or slates, but would also create a current by the velocity of its movements, and supply a certain. and reliable means of ventilation : it would at once reduce the ventilating column from 5 to 1, since the adulteration of the gases beyond the explosive point takes up the great body of air required in fiery mines. It is estimated that 1000 cubic feet of gas requires 30,000 cubic feet of air to render it perfectly safe. Therefore, if the gas was allowed to escape readily, a large amount of this might be dispensed with.