Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/647

 LIGHTHOUSE 625 lamp consuming annually only 170 gallons of oil a light is obtained in the only direction in which great power is required equal in effect to a lamp consuming 800 gallons per annum. Fixed Condensing Lights of Unequal Range which constantly Illuminate the it-hole Horizon. For this purpose the condensing spherical mirror or spherical mirror of unequal area will be found applicable. Revolving or Intermittent Lights for Condensing the Rays into one Sector. The holophote in fig. 55 throws its rays on straight con densing prisms P, each of which spreads the light over the prescribed sectors, while masks M turning horizontally on pivots cut off the light either slowly or suddenly so as to produce a revolving or an intermittent light, both of which condense all the rays uni formly over the one sector. Application of Condensing Principle to Revolving Lights of Unequal Range, which do not Illuminate the whole Horizon. epeat- Repeating Light. Plane mirrors M (fig 56) revolve on an endless g light, chaiu placed outside of the apparatus and alter the direction of the flashes after they pass into the dark arc on the landward side so as to cause the lenses L, L to repe.it their flashes over the sea ward arc which requires strengthening. The condensing spherical mirror and mirror of unequal areas will also be found applicable in cases where the flashes do not require to sweep over the whole horizon. evolv- Condensing Jicvolv- ig ing Lights ivhich Peri- ghts. odically Illuminate the whole Horizon, but u-hich Vary the Strength of the Flashes in Pass- -^ ing over Certain Sec- j tors. The spherical mirror of unequal areas and the condensing FlG - 56. Horizontal Section. mirror are equally well suited for those that revolve. nter- Condensing Intermittent Lights. Figs. 57 and 58 show straight littent refracting or reflecting prisms, which revolve and intercept as they ghts. pass round certain of the rays from a central fixed light apparatus so as to produce perfect darkness over the sectors which they subtend at the time, while they spread the rays which they intercept uniformly over and thus strengthen the interme diate sectors which are illuminated directly by the central apparatus. FIG. FIG. 58. Elevation. Inter mittent light with differen tial re fractor. 57. Horizontal Section. The peculiar property of this arrangement is that the power is increased in proportion to the duration of the intervening periods of darkness. Thus, ne glecting the loss by ab sorption, &c. , the power is doubled when the periods of light and darkness are equal, tre bled when the dark periods are twice as long as the light, and so on in proportion, while in every case the rays are spread uniformly over each illuminated sector. Inter m ittent Condens ing Light with Differen tial Refractor. Figs. 59 and 60 show the new apparatus of Mull of Galloway in which ABA is the differential re fractor, by the com pound horizontal and vertical action of which single agent the whole condensing inter mittent effect is produced, so that condensing prisms are don&amp;lt; Change of fixed to intermit tent ap paratus. way with opposite the central, which is the most important, part f the apparatus. The centre of the inner curve of the refractor is at in fig. 60. , / Though there is no relative mo- ,ion in this ap paratus, every r&amp;gt;art of which re volves together, the parts may be arranged so that the con densers only move. This ap paratus was con structed in the most satisfactory manner by Messrs Barbier & Fenes- tre, Paris. Alteration of Fixed to Intermit tent Apparatus. Any existing fixed light can at once be made in termittent so as to show either equal or unequal periods by simply causing condensing prisms to circulate round it, while the power will be increased in proportion to the ratio of the duration of light to dark periods. Beacons and Buoys. Beacons in exposed situations are constructed sometimes of Bc-acons. stone, and cement-concrete or cement-rubble, but generally of cast- iron columns let into heavy base plates which are fixed to the rock by strong lewis bats. The small class iron beacons are generally of malleable iron and the larger of cast-iron, but steel or bronze might with advantage be used in very exposed places. Fig. 61 shows a first order cast- iron beacon as used in Scotland. T. Stevenson s Apparent Light. This kind of light is specially useful at places where there is a sunk rock with little sea-room round it, and at the pierheads of harbours which must be closely hugged by vessels seeking entrance. It con sists of certain forms of optical apparatus for reflect ing and redistributing at a beacon placed on a sub merged rock parallel rays which proceed from a lamp and apparatus placed on the land. An optical deception is thus produced, as the sailor naturally supposes that there is a lamp burning on the beacon itself. Apparent li- ht. Fig. 61 The first light of this kind, which has been in use since 1851, was placed in a beacon on a sunk reef in Storno- way Bay, and is shown pictorially in fig. 62. This light is 530 feet distant from the lighthouse where the lamp is placed. Others have since been established at Grangemonth, which is 535 feet from the light, and at lesser distances at the harbours of Ayr and Arbroath, at Odessa in the Black Sea, and at Gat Combe Head, Queensland. Beacon Lights. Lamps without glass chimneys, as used in the Beacon early experiments with paraffin, and as used with gas in Pintsch s lights. buoy, having iron tubes placed at certain distances above the flam,?, and supplied with very large cisterns of crystal oil, have been kept continuously burning in Scotland for about a month without trim ming. These lamps arc for rocks at sea which can only be reached when the weather is moderate. Illumination of Beacons by Gas to Produce a Fixed Light. A light near Port-Glasgow has been illuminated by gas since 1861. The tower is about 300 feet from the shore, and the supply and pressure of gas are regulated by self-acting arrangements on the shore. XIV. 79