112 



SCIENCE, 



[Vol. V., No. 105. 



much interesting information on the subject of light- 

 house illuminants in the form of correspondence 

 between the Board of trade, which has general super- 

 vision of the lights of Great Britain; the Trinity 

 house, which manages the English lights; and the 

 Commissioners of northern lights, who have control 

 over those of Scotland. 



It may be remembered that in 1883 it was proposed 

 to make exhaustive tests of the relative value of 

 petroleum, gas, and electricity, as illuminants for 

 lighthouses, by comparing the several lights in actual 

 operation together at the South Foreland station; and 

 the lighthouse authorities of all three kingdoms had 

 arranged to act 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 these 

 would place the system favored by the Irish authori- 

 ties — the Wigham gas system — at a disadvantage, 

 and refused to take further part in the proceedings. 



Dr. Tyndall, who had for years acted as scientific 

 adviser to the Trinity house, but had 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. Wigham's burners, as a lighthouse illuminant; 

 and, further, to imply that the engineer of the board, 

 Mr. (now Sir James) Douglass, has 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 state of 

 development of illuminants adapted to this special 

 purpose, may be taken to be the defence of the board 

 against Dr. TyndalPs strictures. 



From the learned professor's statement, it appears 

 that in 1869, when he was sent to Ireland to make 

 himself acquainted with the gas system of lighthouse 

 illumination, colza-oil was used in the Trinity-house 

 lamps; and this was superseded, at a vast saving to 

 the country, by mineral oil. Mr. Wigham 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-flame could be made to send forth 

 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 alphabet. Further, Mr. Wigham had sur- 

 rounded his central ' bunch ' with rings of burners, 

 to increase the light in thick weather. In a few sec- 

 onds a light-keeper could pass from 28 jets to 48, and 

 thence with equal rapidity to 08, 88, and finally to 108 

 jets, all these flames being under the most perfect 

 control. The best oil-flames then known were feeble 

 scintillations, compared with the flame of the 108- 

 jet burner. Dr. Tyndall adds to his own the tes- 

 timony of many others as to the value of the Wigham 

 system as then examined, and proceeds to describe 

 a later visit to the lighthouse at Galley Head, which 

 is now, he says, without a rival in the world. In 



this light the refracting-lenses of four first-order 

 apparatus are fitted one above another in the same 

 lantern, with a 108-jet burner in the focus of each 

 apparatus. It had already been visited by the Elder 

 brethren of the Trinity house; and their engineers 

 report, he claims, was the only one unfriendly to the 

 light. In spite 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 the doctor, 

 recognized the merits of the gas system, and decided 

 to adopt it, but for the extinction rather than with 

 the co-operation of Mr. Wigham. 



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 opinion 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-jet flame is four 

 inches and a quarter; that of the 48 is five inches 

 and seven-eighths; and so on, until a diameter of 

 eleven inches and an eighth is reached with the 108- 

 jet burner. Then, as the prisms of the optical appa- 

 ratus are adjusted to a focus within the confines of 

 the small 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 appara- 

 tus. This effect is not particularly important in a 

 fixed light showing all around the horizon. By far 

 the greater number of fixed lights, however, require 

 to be either strictly confined in angular width, or 

 marked with color within particular bearings, which 

 is accomplished by interposing fixed vertical screens, 

 opaque or of colored glass, close to the glazing of 

 the lantern. Directly the diameter of the flame is 

 enlarged, the screen will no longer cut off the light 

 with precision on its appointed bearings: the ex-focal 

 rays of white 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 Wigham burner, — 

 for instance, the six-wick oil-burner, four inches and 

 three-eighths wide, being equal in power to the 48-jet 

 gas, five inches and seven-eighths wide, — it follows 

 that oil is, according to the facts before us, more 

 suitable for important niceties of direction. Occul- 

 tation — that is, the sudden and short eclipse, at 

 regular intervals, of an otherwise continuous light — 

 is effectively applied with either source of illumina- 

 tion, but in the Wigham system is applied to flashing 

 lights in a novel manner, as an. additional means of 

 identification. A light showing one long flash every 

 minute, is, by occultation at short intervals, made to 

 show a number of short flashes instead of the long 

 one. With a widening burner, the luminous beam 

 becomes broader, and the number of flashes seen in 

 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 flashes 

 had been observed. 



