UNWATCHED AND UNATTENDED LIGHTS Electric.—Since 1884, when an iron beacon lighted by an incandescent-electric lamp supplied with current from a sec ondary battery was erected on a tidal rock near Cadiz, various forms of electric unattended lights have been experimented with. In 1928 many such lights were in permanent use where current could be obtained from a local supply circuit. Storage batteries, small automatic generating sets, and stand-by gas burners are alternatively provided to guard against the failure of the main supply. Automatic devices are also fitted for changing one lamp for another if the lamp in focus should fail, and for switching in the stand-by apparatus when necessary. Both revolving and fixed optics are used. In the former case the optic is rotated by a small electric motor and in the latter some form of automatic interrupter produces the flashing characteristic. In cases where the optic is too small for a burner changer to be accommodated, two lenses are employed, one superimposed on the other. In one of the lenses is focussed the electric lamp in normal use and in the other the stand-by gas or electric lamp. An example of sec ondary lights, formerly attended, and now converted to electric, is at Burnham in Somerset where two leading lights over 4 m.
apart were so altered in 1928, and are now unwatched at night. In this case the optics (see Plate II.) are of fixed section, and the automatic features include a flashing device for giving the lights their respective characteristics.
Other Forms of Beacon Lighting.—Among other systems of unattended beacon lighting adopted in lighthouse services since about 188o, but now little used, may be mentioned the Lindberg light, a Swedish invention employing a volatile spirit ; the Benson Lee lamp having a carbon-tipped wick; the French permanent wick lamp; and the Wigham lamp in which a flat wick immersed in petroleum travelled over a horizontal roller so that the petroleum was volatilised from a constantly renewed surface. Oil gas, mostly in its modified forms is still largely employed.
Acetylene Lights.—The gas is provided either by an auto matic generator or else in the form of compressed acetylene dis solved in acetone. In order to reduce the consumption and at the same time give a distinctive characteristic to the light, the gas is usually passed through an automatic flashing mechanism which works continuously until the supply is exhausted. Waste
of gas during daylight is sometimes obviated by using a sun-valve, which is a device to turn off the gas at daybreak and turn it on at dusk. Sun-valves depend for their automatic action on the differential expansion of two distinct metals under the influ ence of light rays or on the difference in the absorption of light rays by black and bright bodies respectively. The movement of a lever arm, brought about by a small movement of the light sensitive element, actuates a valve which opens and closes the gas supply. Acetylene in unattended lights is burnt either as an open flame or in conjunction with a mantle.
The "Aga" system of acetylene lighting, developed in Sweden about 1904, has been extensively used both in that and other countries for all classes of unattended lights, including buoys.
It was first adopted in England in 1913. Other systems embody ing similar principles are also in use. The "Dalen" incandescent mantle burner embodies a mix-flasher which automatically con trols the character of the light and regulates the mixing of air and gas for consumption in an inverted soft-mantle burner.
In some unattended lights with acetylene illumination the lenses are rotated by a gas pump as the gas passes to the mixer and burner, the lens table moving on ball bearings or a mer cury float; the sun-valve automatically controls the duration of lighting; a pilot jet serves to re-ignite the main burner at sun set; and an automatic changer provides for the replacement of a broken mantle. Incandescent mantle burners and acetylene equip ment have also been installed in a number of lighthouses of secondary importance, formerly attended by keepers, and now converted to unwatched or semi-watched lights. Among such in England are the lights at the Rock (Liverpool) ; Great Orme Head and St. Tudwall (Carnarvonshire) ; Berry Head (S. Devon) ; Peninnis Head (Scilly Isles) ; East Usk (Mon.) ; and Great Castle Head (Pembrokeshire). The Menai lighthouse has been fitted with an open-flame acetylene equipment. In some cases, where the consumption of gas is large, acetylene generators on the carbide-to-water system have been installed, as at the Inner Fern and Bamburgh lights on the coast of Northumber land, and the two first-order leading lights at Hurst (Plate I., fig. 3), on the mainland opposite the Isle of Wight.