. Azimuthal Condensing above is a description of the general principles on which light-houses are illuminated. In placing a light in some situations, regard, how ever, must be had to the physical peculiarities of the localities; the following plans of Mr. Thomas Sfevenson may be cited as examples. In fixed lights of the ordinary con struction, the light is distributed, as already explained, equally all round the horizon, and is well adapted for a rock or island surrounded by the sea. But where it is only necessary to illuminate a narrow sound, it is obvious that the requirements are ve,ry different. On the side next the shore, no light is required at all; across the sound, .a. feeble light is all that is necessary, because the distance at which it has to be seen is !. small, owing to the narrowness of the channel; while up the sound and down the sound ' the sea to be illuminated is to be of greater or lesser exteni, and requires a corresponding intensity. If the light were made sufficiently powerful to answer for the greater dis tance, it would be much too powerful for the shorter distance across the sound. Such an arrangement would occasion an unnecessary waste of oil, while the light that was cast on the landward side would be altogether useless. Fig. 3 represents (in plan) the condensing light, by which the light proceeding from the flame is allocated in the different azimutla zn proportton to the distances at whichthe light requires to be seen by the mariner in those azimuths. Let us suppose that the rays marked a require to be seen at the.greatest distance down the sound, and those marked fi to a •somewhat hi.. u 'NATE° Apc ,..
smaller distance up the sound. ..V ..
...• In order to strengthen those arcs, -,,.
the spare light proceeding laud- •.
.
wards, which would otherwise be lost, is intercepted by portions ....
of holophotes, B and 0, sub- i a i \\ 1/// •• tending spherical angles propor- , S , tioned to the relative ranges and ' • a_.,_----- angular spaces of the arcs a and , 4040=4,..W• fi. The portions of light thus ,-_______ .1%, 11/4‘V___ ------.
L intercepted are parallelized by -,7----- • g \ • ..........6 ,ap.
---- _....... Ir the holophotes, and fall upon 40.A.V41=-,----' "‘"-Ir...-0- .q. • straight prisms a, a, and b, b, ,, 7---1j,,, 1040: f „%,'"Z4,41,Ay ...17.
respectively, which again refract ..----- - gase .r WA,/ / hip247c; .
\ thein in the horizontal plane ..._ •-•-•••••_.4, 14/1,0,0"/ No-,-- only ; and, after passing through -.......... .,... .,* ..,- ..;„._;___,_ focal points (independent for v-t:.•-•- --- '-'• Ce"...,-.40/40i • ; ' '„....--- .----'7' ---,7-' each prism), they emerge in N separate equal beams,ancl diverge through the same angles as a \\\_ .,,,,,04,...-- ..
and /3 respectively. In this way, -,.
the light proceeding up find down '‘.
the sound is strengthened in the "....„....
.... .......
required ratio by utilizing, in the -DARK ANG-rn manner we have described, the Fig. 3.
light which would otherwise have been lost on the land. These instruments were first introduced at three sound lights in the w. of Scotland, in 1857, where apparatus of a small size, combined with a small burner, was found to produce, in the only directions in which the great power was required, beams of light equal to the largest class of apparatus and burner. The saving thus effected in oil, etc., has been estimated at about £400 or £500 per annum for these three stations.
Apparent Light. —At Stornoway bay, the position of a sunk rock has been sufficiently indicated by means of a beam of parallel rays thrown /runt the shore upon certain optical apparatus fixed in the top of a beacon erected upon the rock itself. It was suggested that the light-house should be built on the outlying submerged reef, but the cost would have been very great, and Mr. Stevenson's suggestion of the apparent light was adopted. By means of this plan the expense of erecting a light-house on the rock itself
has been saved, and all the purposes of the mariner served. It has been called an apparent light from its appearing to proceed from a flame on the rock, while the light in reality proceeds from the shore, about 650 ft. distant, and is refracted by glass prisms placed on the beacon. .
Floating lights are vessels fitted with lights moored at sea in the vicinity of reefs. Prior to 1807 the lantern was hung at the yard-arin. The late Mr Robert Stevenson then introduced the present system of lanterns, having a copper tube in the center capa ble of receiving the vessel's mast, which passed through the tube, the lights being ' placed all round. In this way proper optical appliances can be employed, and the lan tern can be 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 Hoogly under •the directions of Messrs. Stevenson, in which the dioptric principle was applied. Eight half-fixed light apparatus of glass with spherical mirrors behind. were placed in the lantern round the mast, so as to show in every azimuth rays from three of them at once.
Differential Lens.—This is an annular lens, curved to different radii on both sides, so as to increase the divergence in any given ratio. The small arc of about 6° which is unequally illuminated by the lens as presently constructed, ling be made o'f equal intensity throughout by the differential form, or by means of separate straight prisms placed at the sides.
Sources of Light.—The descriptions which have already been given have all had reference to the best means of employing a given light. Many attempts have from time to time been made to increase the power of the radiant itself.
Magneto-electric Light.—The electric light, which has of late been greatly developed and improved, and especially adapted to light-house purposes, was introduced under the auspices of the Trinity house of London.
Gas.—The uncertainty and other objections attending the manufacture and use of gas in remote and inaccessible places have, with soine exceptions, as yet prevented its adoption at light-house stations, but it has been successfully used at many harbor-lights.
Oil and Paraffine.—The oil which is chiefly employed in Great Britain is that which goes by the naine of colza, and the quantities annually consumed at the northern light houses may be stated at 40 galls. for an argand 1 in. in diameter, and 800 galls. for the four-wick burner, which is used in dioptric lights of the first order. Capt. Doty's burner for paraffine, which is the best which has as yet been suggested; has been introduced into the French and the Scotch light-lionses. Paraffine has been found to give a more intense light than colza at half the cost.
Visibility of Lights.—The distance at which any light can be seen, of course depends on the height of the tower, and varies with the state of the atmosphere. The greatest recorded distance at which an oil light has been visible is that of the holophotal light of Allepey at Travancore, which has been seen from an elevated situation at a distance of 45 miles. The Iiolophotal revolving light at Baccalieu, in Newfoundland, is seen every night in clear weather at cape Spear, a. distance of 40 nautical miles.
Power of Light-house Apparatus.—The reflector (25 in. diam.) used in the northern light-houses, with a burner of 1 in. diam. is considered equal to about 360 argand flames. The cylindric refractor, used in 'fixed lights, with a four-wick burner, has ban estimated at 250; while the annular lens in revolving lights, with the same burner, is equal to about 3,000 argand flames. See LIGHTING OF BEACONS Axn BUOYS AT SEA.