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 whole 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 condensing 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 uniformly over the one sector.

Application of Condensing Principle to Revolving Lights of Unequal Range, which do not Illuminate the whole Horizon.

Repeating light.

Repeating Light. – Plane mirrors M (fig 56) revolve on an endless chain 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 repeat their flashes over the seaward 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.

Revolving lights.

Condensing Revolving Lights which Periodically Illuminate the whole Horizon, but which Vary the Strength of the Flashes in Passing over Certain Sectors. – The spherical mirror of unequal areas and the condensing mirror are equally well suited for those that revolve.

Fig. 56. – Horizontal Section. Fig. 57. – Horizontal Section. Fig. 58. – Elevation.

Intermittent lights.

Condensing Intermittent Lights. – Figs. 57 and 58 show straight refracting or reflecting prisms, which revolve and intercept as they 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 intermediate sectors which are illuminated directly by the central apparatus. The peculiar property of this arrangement is that the power is increased in proportion to the duration of the intervening periods of darkness. Thus, neglecting the loss by absorption, &c., the power is doubled when the periods of light and darkness are equal, trebled 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.

Intermittent light with differential refractor.

Intermittent Condensing Light with Differential Refractor. – Figs. 59 and 60 show the new apparatus of Mull of Galloway in which ABA is the differential refractor, by the compound horizontal and vertical action of which single agent the whole condensing intermittent effect is produced, so that condensing prisms are done away with opposite the central, which is the most important, part of the apparatus. The centre of the inner curve of the refractor is at O in fig. 60. Though there is no relative motion in this apparatus, every part of which revolves together, the parts may be arranged so that the condensers only move. This apparatus was constructed in the most satisfactory manner by Messrs Barbier & Fenestre, Paris.

Change of fixed to intermittent apparatus.

Alteration of Fixed to Intermittent Apparatus. – Any existing fixed light can at once be made intermittent 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.

Beacons in exposed situations are constructed sometimes of 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.

Apparent light.

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 consists of certain forms of optical apparatus for reflecting and redistributing at a beacon placed on a submerged 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. The first light of this kind, which has been in use since 1851, was placed in a beacon on a sunk reef in Stornoway 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.

Fig. 59. Fig. 60. Fig. 61.

Beacon lights.

Beacon Lights. – Lamps without glass chimneys, as used in the early experiments with paraffin, and as used with gas in Pintsch's buoy, having iron tubes placed at certain distances above the flame, and supplied with very large cisterns of crystal oil, have been kept continuously burning in Scotland for about a month without trimming. These lamps are 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.