The early open-flame lamps not only were sources of danger but their feeble varying intensity caused serious damage to the eyesight of miners. This factor is always present in inadequate and improper lighting, but its influence is noticeable in coal-mining in the nervous disease affecting the eyes which is known as nystagmus. The symptoms of the disease are inability to see at night and the dazzling effect of ordinary lamps. Finally objects appear to the sufferer to dance about and his vision is generally very much disturbed.
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The oil-lamps used in coal-mining have a luminous intensity equivalent to about one to four candles, but owing to the atmospheric conditions in the mines a flame does not burn as brightly as in the fresh air. The possibility of explosion due to the open flame was eliminated by surrounding it with a metal gauze. Davy was the inventor of this device and his safety lamp introduced about a hundred years ago has been a boon to the coal-miner. Various improvements have been devised, but Davy’s lamp contained the essentials of a safety device. The flame is surrounded by a cylinder of metal gauze which by forming a much cooler boundary prevents the mine-gas from becoming heated locally by the lamp flame to a sufficient temperature to ignite and consequently to explode. This device not only keeps the flame from igniting the gas but it also serves as an indicator of the amount of gas present, by the variation in the size and appearance of the tip of the flame. However, the gauze reduces the luminous output, and as it accumulates soot and dust the light is greatly diminished. One of these lamps is about as luminous as a candle, initially, but its intensity is often reduced by accumulations upon the gauze to only one fifth of the initial value.
The acetylene lamp is the best open-flame light-source available to the miner, for several reasons. It is of a higher candle-power than the others and as it is a burning gas, there is not the danger of flying sparks as in the case of burning wicks. The greater intensity of illumination affords a greater safety to the miner by enabling him to detect loose rock which may be ready to fall upon him. However, this lamp may be a source of danger, owing to the fact that it will burn more brilliantly in a vitiated atmosphere than other flame-lamps. Another disadvantage is the possibility of calcium carbide accidentally spilt coming in contact with water and thereby causing the generation of acetylene gas. If this is produced in the mine in sufficient quantities it is a danger which may not be suspected. If ignited it will explode and may also cause severe burns.
The electric lamp, being an enclosed light-source capable of being subdivided and fed by a small portable battery, early gave promise of solving the problem of a safe mine-lamp of adequate candle-power. Much ingenuity has been applied to the development of a portable electric safety mine-lamp, and several such lamps are now approved by the Bureau of Mines. Two general types are being manufactured, the cap outfit and the hand outfit. They consist essentially of a lamp in a reflector whose aperture is closed with a sheet or a lens of clear glass. The battery may be of the “dry” or “storage” type and in the case of the cap outfit the battery is carried on the back. The specifications for these lamps demand that a luminous intensity averaging at least 0.4 candle be maintained throughout twelve consecutive hours of operation. At no time during this period shall the output of light fall below 1.25 lumens for a cap-lamp and below 3 lumens for a hand-lamp. Inasmuch as these are equipped with reflectors, the specifications insist that a circle of light at least seven feet in diameter shall be cast on a wall twenty inches away. It appears that a portable lamp is an economic necessity in the coal-mines, on account of the expense, inconvenience, and possible dangers introduced by distribution systems such as are used in most places.
Although the major defects in lighting are due to absence of light in dangerous places, to glare, and to other factors of improper lighting, there are many minor details which may contribute to safety. For example, low lamps are useful in making steps in theaters and in other places, in drawing attention to entrances of elevators, in lighting the aisles of Pullman cars, under hand-rails on stairways, and in many other vital places. A study of accidents indicates that simple expedients are effective preventives.