[1] A full account is given in Hermann Thiersch, _Pharos Antike,
Islam und Occident_ (1909). See also MINARET.
[2] In 1901 one of the lights decided upon in 1886 and installed in
1888--Créac'h d'Ouessant--was replaced by a still more powerful twin
apparatus exhibited at the 1900 Paris Exhibition. Subsequently
similar apparatus to that at Créac'h were installed at Gris-Nez, La
Canche, Planier, Barfleur, Belle-Île and La Coubre, and the old
Dunkerque optic has been replaced by that removed from Belle-Île.
[3] Both the Talais and Snouw light-vessels have since been converted
into unattended light-vessels.
[4] For the purposes of the mariner a light is classed as flashing or
occulting solely according to the duration of light and darkness and
without any reference to the apparatus employed. Thus, an occulting
apparatus, in which the period of darkness is greater than that of
light, is classed in the Admiralty "List of Lights" as a "flashing"
light.
[5] The Flamborough Head rocket was superseded by a siren fog signal
in 1908.
LIGHTING. Artificial light is generally produced by raising some body to
a high temperature. If the temperature of a solid body be greater than
that of surrounding bodies it parts with some of its energy in the form
of radiation. Whilst the temperature is low these radiations are not of
a kind to which the eye is sensitive; they are exclusively radiations
less refrangible and of greater wave-length than red light, and may be
called infra-red. As the temperature is increased the infra-red
radiations increase, but presently there are added radiations which the
eye perceives as red light. As the temperature is further increased, the
red light increases, and yellow, green and blue rays are successively
thrown off. On raising the temperature to a still higher point,
radiations of a wave-length shorter even than violet light are produced,
to which the eye is insensitive, but which act strongly on certain
chemical substances; these may be called ultra-violet rays. Thus a very
hot body in general throws out rays of various wave-length; the hotter
the body the more of every kind of radiation will it throw out, but the
proportion of short waves to long waves becomes vastly greater as the
temperature is increased. Our eyes are only sensitive to certain of
these waves, viz. those not very long and not very short. The problem of
the artificial production of light with economy of energy is the same as
that of raising some body to such a temperature that it shall give as
large a proportion as possible of those rays which the eye is capable of
feeling. For practical purposes this temperature is the highest
temperature we can produce. As an illustration of the luminous effect of
the high temperature produced by converting other forms of energy into
heat within a small space, consider the following statements. If burned
in ordinary gas burners, 120 cub. ft. of 15 candle gas will give a light
of 360 standard candles for one hour. The heat produced by the
combustion is equivalent to about 60 million foot-pounds. If this gas be
burned in a modern gas-engine, about 8 million foot-pounds of useful
work will be done outside the engine, or about 4 horse-power for one
hour. If this be used to drive a dynamo for one hour, even if the
machine has an efficiency of only 80%, the energy of the current will be
about 6,400,000 foot-pounds per hour, about half of which, or only
3,200,000 foot-pounds, is converted into radiant energy in the electric
arc. But this electric arc will radiate a light of 2000 candles when
viewed horizontally, and two or three times as much when viewed from
below. Hence 3 million foot-pounds changed to heat in the electric arc
may be said roughly to affect our eyes six times as much as 60 million
foot-pounds changed to heat in an ordinary gas burner.
Owing to the high temperature at which it remains solid, and to its
great emissive power, the radiant body used for artificial illumination
is usually some form of carbon. In an oil or ordinary coal-gas flame
this carbon is present in minute particles derived from the organic
substances with which the flame is supplied and heated to incandescence
by the heat liberated in their decomposition, while in the electric
light the incandescence is the effect of the heat developed by the
electric current passed through a resisting rod or filament of carbon.
In some cases, however, other substances replace carbon as the radiating
body; in the incandescent gas light certain earthy oxides are utilized,
and in metallic filament electric lamps such metals as tungsten or
tantalum.