Coal-gas was introduced into lighthouses in about the middle of the nineteenth century. Inasmuch as the gas-mantle had not yet appeared, the gas was burned in jets. Various arrangements of the jets, such as concentric rings forming a stepped cone, were devised. The gas-mantle was a great boon to the mariner as well as to civilized beings in general. It greatly increases the intensity of light obtainable from a given amount of fuel and it is a fairly compact bright source which makes it possible to direct the light to some degree by means of optical systems. Owing to the elaborate apparatus necessary for making coal-gas, several other gases have been more desirable fuels for lighthouse lamps. Various simple gas-generators have been devised. Some of the high-flash mineral-oils are vaporized and burned under a mantle. Acetylene, which is so simply made by means of calcium carbide and water, has been a great factor in lighting for navigation. By the latter part of the nineteenth century lighthouses employing incandescent gas-burners were emitting beams of light having luminous intensities as great as several hundred thousand candles. These special gas-mantle light-sources have brightness as high as several hundred candles per square inch. Electric arc-lamps were first introduced into lighthouse service in about 1860, but these lamps cannot be considered to have been really practicable until about 1875. In 1883 the British lighthouse authorities carried out an extensive investigation of arc-lamps. It was found that the whiter light from these lamps suffered a greater absorption by the atmosphere than the yellower light from oils, but the much greater luminous intensity of the arc-lamp more than compensated for this disadvantage. The final result of the investigation was the conclusion that for ordinary lighthouse purposes the oil-and gas-lamps were more suitable and economical than arc-lamps; but where great range was desired, the latter were much more advantageous, owing to their great luminous intensity. Electric incandescent filament lamps have been used for the less important lights, and recently there has been some application of the modern high-efficiency filament lamps.