Page:Science vol. 5.djvu/128

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��tnucb iiilereslinf; lufonnatioii on the subject of liglit- Louse llluminaiits in the fonn of correapondence belwe«ii the Board of trade, which has general super- viaion of the lights of Greftt Britain; the Trinity house, which manages Ihe English lights; and the Commission era of uorthcni lights, who Imve coulrol over those of Scotland.

It may he remembereJ that in 1883 It was proposed 1.0 make exhaustive tests of the relative value of petroleum, gas, aud electricity, as llliiminants for liglitbousei, \ij comparing the several lights in actual uperationtogeLherat the SouCb Foreland statloa; and the llghtliouse authorities of all three kingdoms bad arranged to set conjointly in prosecuting the ex- periments. When, however, the conditions under which the trials were to take place were formulated, the representatives from Ireland considered that tiiesa would place the system favored by the Irish authori- ties — the WIgham gas system — at a disadvantage, lind refused to take further part in the proceedings.

Dr. Tyndall, who had for years acted as scientific adviser Co the Trinity house, but iiad prior to this re- signed, then wrote certain letters to the newspapers on the subject. These letters appear, says the Board of trade, to assert the superiority of gas, as used in Mr. Wigharn's buruurs, as a lightbouse lllumiiistil; and, further, to imply that the engineer of the board, Mr. (now Sir James) Douglass, baa not been entirely disinterested. The Board of trade therefore asked for a full report of the views of the English and Scotch lighthouse boards on the whole question; and their replies, which give a fair Idea of the present slate of development of llluminanis adapted to this special purpose, may be taken to be the defence of llie board against Dr. Tyndall's strictures.

From the learned professor's statement, it appears that 111 18011, when he was sent to Ireland to make himself acguainled with the gas system of tlghtbouae illumination, colza-oil was used in the Trinity-house lamps; and tills was superseded, at a vast saving to the country, by mineral oil. Mr. Wlgliam had suc- ceeded in producing a gas-lamp superior in power to the best oil-lamp then extant. The gas-flame showed a promptitude of action and a pliancy of adaptation unattainable with oil. By a simple automatic ap- paratus, the gas-flaine could tw ninde to send forlb flashes in any desired succession, and of any required duration. Long and short flashes could be combined so as to render the Identity of a lighthouse unmis- takable, or enable It to spell its own name by the Morse alphat>et. Further, Ur. Wigham Iiad sur- rounded his central ' bunch ' with rings of btuners, to Increase the light in thick weather. lu a few sec- onds a light- keeper could jiaes from 3S jets lu 48, and thence with equal rapidity to S8, 88, and finally to 108 jets, alt these flumes being under the mosl perfect control. The best oil-flames then known were feeble scintillations, compared with the flame of the 108- jct burner. Dr. Tyndall adds to bia own the tes- timony of many others as to the value of the Wigham system as then examined, and procee<i« to describe a later visit to Ibe iigbtliouse at Galley Head, which is now, he says, without a rival in the world. In

��this ligiil the refract ing-lenses of four flrst-order appanttus are fitted one above another in the soma lantern, with a l(S-jct burner lu the focus of each apparatus. It had already been visited by the Elder brethren of the Trinity house; and their engineer's report, he claims, was the only one unfriendly to the light. In spile of the almost unanimous opinion in its favor, the Trinity house decided in favor of a six- wick burner consuming mineral oil (Sir James Doug- lass's patent). Finally, Sir James, says Ihe doctor, recognized the merits of the gas system, and decided to adopt It, but for the exllnctlnn rather than with the co-operation of Mr. Wigbam.

The Trinity house replies at considerable length, giving in full the result of its Investigations into the worth of the Wigham light. From these obser- vations, the Elder brethren derived an opiuion that one prominent objection to It Is, that the higher powers of the single burner are obtained by increas- ing Its size. The diameter of the 28-jeI flame Is four inches and a quarter; that of the 46 is five inches and seven -eighths; and so on, until a diameter of eleven inches aud an eighth is reached with the lOB- jet burner. Then, as the prisms of the optical appOr ralus are odjusled to a focus within llie confines of the email flame, it follows that a great portion of the enlarged flame is extra-focal, and distributed In di- rections not Intended by the designer of the apparor tus. This effect Is nut particularly important in a Bsed light showing all around the horizon. By (ai the greater number of fixed lights, however, require to be either alrlctly confined in angular width, or marked with color within particular bearings, which is Bccompiistied by iiilerposing fixed vertical screens, opaque or of colored gloss, close to the glazing of the lantern. Directly the diameter of the flame is enlarged, the screen will no longer cut oS the light with precision on iLs appointed bearings: the ex-focal rays of while light will stray into the sector which should be dark or colored, and destroy the means of guidance for which the light is intended.

The diameter of the oil-burner being constant, and its flame more compact than the Wigbam burner, — for Instance, the slx-wick oil'burner, four inches and thrce-elghlhs wide, being e<jual In jMiwer lothe iS-jet gas, five inches and seven-eighths wide, — it follows that oil is, according to the facts before suitable for important niceties of direction. Occnl- tation — that is, the sudden and short eclipse, aX regular Intervals, of on otherwise continuous light - is effectively applied with either source of illiunlns- tlon, butlu the Wigham system Is applied to lights in a novel manner, as an additional i identification. A light showing one long flash every minute, is, by occultatlon at short intervals, made t show a numl^or of short flashes Instead of the long one. With a widening burner, the luminous f becomes broader, and the number of flashes se each series becomes greater; so that the expansion of a burner involves a change in that distinctive charac- ter upon which the observer most relies. At Galley Head this uncertainty as to the number of f liad lietn observed.

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