.

Naturally, mineral oil was introduced as soon as it was available, owing
to its lower cost; but it was not until nearly 1870 that a satisfactory
mineral-oil lamp was in operation in lighthouses. Doty is credited with
the invention of the first successful multiple-wick lighthouse lamp
using mineral oil, and his lamp and modifications of it were very
generally used until the latter part of the nineteenth century. These
lamps are of two types--one in which oil is supplied to the burner under
pressure and the other in which oil is maintained at a constant level.
In some of the smallest lamps the ordinary capillarity of the wick is
depended on to supply oil to the flame.

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 inv