Page:The New International Encyclopædia 1st ed. v. 12.djvu/285

* LIGHTHOUSE. 259 LIGHTHOUSE. ward side would be altogether useless. Fig. 5 represents (in plan) the condensing light, by which the light proceeding froth the flame is allocated in the different azimuths in proportion r(Zfea', "^"-h Arc Fie. 6. PI.AK OF CONDENSING LIQHT. to the distances at which the light requires to be seen by the mariner iu those azimuths. Let us suppose that the rays marked a require to be seen at the greatest distance do^'n the ^iound. and those marked 6 to a somewhat smaller distance up the sound. In order to strengthen those arcs, the spare light proceeding landward, which would otherwise be lost, is intercepted by por- tions of holophotes, B and C, subtending spheri- cal angles proportioned to the relative ranges and angular spaces of the arcs a and h. The portions of light thus intercepted are made parallel by the holophotes, and fall upon straight prisms aa and bb respectively, which again re- fract them in the horizontal plane only; and, after passing through focal points (independent for each prism), they emerge in separate equal beams, and diverge through the same angles as a b respectively. In this way, the light pro- ceeding up and down the sound is strengthened in the required ratio by utilizing, in the manner we have described, the light which would other- wise have been lost on the land, and a consider- able saving is effected. LlGUT.suiPS arc vessels fitted out with lights moored at sea in the vicinity of reefs. Prior to 1807 the lantern was hung at the yard-arm. Kobert Stevenson then introduced a system of lanterns, grouped around a copper tube capable of receiving the vessel's mast, which passed tlirnugh the tube. In this way proper optical appliances could be employed, and the lantern lowered on the mast so as to pass through the roof of a house on the deck, where the lamps are filled or trimmed. In 1864 six floating lights were constructed for the Hugli under the direc- tions of the Messrs. Stevenson, in which the diop- tric principle was applied. Eight half-fixed light apparatus of glass with spherical mirrors be- hind were placed in the lantern round the mast so as to show in every azimuth rays from three of them at once. In 1900 the United States had 44 lightships in position, each ship showing either one or two lights. Each light is com- posed of eight reflectors li iaclies in diameter set upon a ring which encircles the mast and can be lighted and lioistcd to the masthead at night and lowered and housed by day. Certain of the British lightships show revolving lights, some of which are red. Lightships are employed in those positions where lighthouses are necessary but where thej- have not been constructed because of the great difficulty of erecting them. Sources of Ligut. The descriptions which have already been given have all had reference to the best means of employing a given light, ilany attempts have from time to time been made to increase the power of the radiant itself. The illuminant of the lighthouse establishment has been changed frequently. The earliest one used on the Xorth American coast consisted of balls of pitch and oakum burned in open braziers at Point Allerton, in 1673, which was succeeded by tallow candles at Boston Harbor in 1716. Xext came fish oil, burned in spider lamps, and afterwards sperm oil. burned in a sort of argand lamp in W'inslow's magnifying and reflecting lanterns, which remained in u.se up to the time of the establishment of the Lighthouse Board. As sperm oil became more and more ex- pensive, the attention of the board was directed to finding cheaper illuminant, and it was found that colza, the oil expressed from the seeds of wild cabbage and several other plants, which was largely used in France and tireat Britain, would fulfill all the conditions except that of being of home production. This was overcome by stimulating the cultivation of the plants and the manufacture of the oil from the seeds as a private industry. Further experiments with lard oil were made, with such success as to prove that the latter oil of a certain grade was a more desirable illuminant than colza, as it was more certain in quantity and production and was more economical in price. This be- came the next established illuminant. The u.se of petroleum had attracted attention for a long time, and in 18.55 the United States Light- house Board made some unsuccessful experiments with it. Jleantime the price of lard oil had so increased that some other illuminant became a necessity, and as mineral oil in one form or an- other was in successful use in European light- houses, the board renewed its experiments with petroleum. The first difficulty was that of the lamp in which it should be burned. This was finally overcome by the board, which succeeded in producing a lamp in its own laboratory that proved satisfactory and that was introduced into the lightliouses. The following qualities were determined by the board in reference to petroleum : It fixed the flashing test of the mineral oil that would be accepted for light- house use at 140° F., the fire-test at 154°, and the freezing test, at which it would remain limpid, at zero. Litmus paper inunerscd in it for five hours must, by remaining unchanged in color, show its freedom from acid; its specific gravity must not he less than .802; and it is to be paid for bv weight, at the rate of S-IO pounds net weight to the gallon. The difficulty of .storing and transporting such quantities in bulk was conceded, but its danger was evaded by having the oil placed at once in five-gallon cans, where it was to remain until transferred to the light- house burners for combustion. Mineral oil is now used throughout the lighthouse establish-