Page:The American Cyclopædia (1879) Volume X.djvu/461

 LIGHTHOUSE 455 getting the proper shape of flame for devia- tion by the illuminating apparatus; and the uncertainty of the supply where the gas is made at the lighthouse is another objection, and one so serious that a full supply of oil has had to be kept at the station for fear of acci- dents to the gas apparatus. In a few cases gas has been introduced into lighthouses near towns supplied with it. These, however, have been small lighthouses, and the examples are from the nature of the .case rare, and cannot be extended beyond harbor lights. In one case in the United States a lighthouse is lighted with natural gas. In the present state of the gas manufacture it seems impossible to make a burner that will give the proper size and shape of flame for the large orders of lights. No metal but a very refractory one can bear the intense heat developed by four cylindrical concentric flames, the largest of which is 3^ in. in diame- ter ; and the expense of making burners of such a metal would be very great. As an economi- cal question it is doubtful whether gas should be substituted for oil. The first cost of the gas apparatus for a large lighthouse is heavy, and the annual expense of repairs is also large. It is probable that the efficiency or brilliancy of the light would not be materially increased by the change from oil to gas, as the lights fitted with Fresnel apparatus now show as far as the curvature of the earth will permit. Experi- ments have been made in the United States on the electric light by Prof. Henry, chairman of the lighthouse board. He came to the conclu- sion that the slight gain in its power of pene- trating fogs does not compensate for the in- creased cost of apparatus, difficulty of atten- dance, and liability to derangement. In a lec- ture delivered March 9, 1860, before the royal institution, Prof. Faraday spoke of the light produced by electricity as being especially adapted for lighthouses on account of its in- tensity, while it occupied at its source no more space than a common candle. The voltaic bat- tery, however, presented difficulties which ren- dered its use for this purpose impracticable, but from which the magneto-electric apparatus was found to be free. A large apparatus of this character, worked by a two-horse steam engine, had been employed for six months at the South Foreland lighthouse, which produced such an intensity of light that it was often seen from the opposite coast of France. Prof. Fara- day anticipated that, if the expense of this mode of illumination did not prove too serious, it would be adopted in many situations where intense light is required. Experiments have been made tfpon kerosene with reference to its use in lighthouses, but it is not feasible to make the flame of this oil of the proper size and shape for deviation by the large lenses, in the present state of knowledge on the subject. ' The difficulty appears to be that the immense heat of so large a flame sets free a great amount of carbon, which passes off unconsumed as smoke, and covers the apparatus and glass of the lantern with a thick coating of black. Until means can be devised for the consump- tion of this excess of carbon, kerosene cannot be used. Experiments lately made in the Uni- ted States have convinced the lighthouse board that petroleum is too dangerous a fluid to be used for lighthouse purposes. The danger con- sists not merely in burning it in a proper lamp, but in its transportation in large quantities, in drawing it from the butts, and in filling the lamp reservoirs. In Europe, however, petro- leum is coming into general use, and France has ordered a change to mineral oil in all of its lighthouses. The illuminating apparatus is either catoptric by reflectors, or catadioptric by lenses. The latter method of illumination has been fully described in the article FRESNEL. In the catoptric method, which was mostly used until within the last 30 years, the light from each lamp is so deviated by a reflector that it emerges from the lantern, a beam, or nearly a beam, parallel to the horizon. The ear- liest known instance of a reflector being used for this purpose was in the Cordouan light- house, in the bay of Biscay. With the introduc- tion of the Argand burner its use became more common. The reflectors at first were plaster moulds made of the proper form, upon the in- terior surface of which were fastened facets of plane silvered glass. They came into gene- ral use in Europe in the early part of the pres- ent century. The best form of reflector is the paraboloid of revolution with its axis horizontal. The reflector is made of copper, and its inner surface is covered with silver and is highly pol- ished. The flame of the lamp has its centre in the focus of the reflector, and the rays emerge from the surface of the reflector nearly parallel. They are not entirely parallel, because the sur- face is necessarily imperfect, and the source of light cannot be a mathematical point. The small divergence, instead of being a defect, is in reality a benefit, for without it the beam would always have a diameter equal to that of the edge of the reflector, and would be of little practical value. In fixed lights, the reflectors are fastened to circular iron frames, and are placed in horizontal tiers in the lantern. There is a lamp for each reflector, and it follows that the greater the number of lamps, the more uni- formly the light is distributed around the hori- zon. The reflectors vary in their sizes. The double ordinate at the lips is about 11 in. in the smallest and 21 in. in the largest size. Some have been made larger, but they have never been in general use. In a revolving reflector light, the reflectors are generally arranged so that the axes of all of them on one face are parallel, and there are two, three, or four faces, the number depending on the desired interval between the flashes. The frame upon which they are placed is made to revolve by a clockwork arrangement moved by a weight. It is evident that the flash pro- duced by one of the faces will be brighter than the light of a fixed reflector light, because the