May 30, 1889] 



NATURE 



113 



be produced in the arc, and about 150 millions of candle units 

 in the condensed flashes from the optical apparatus of the dimen- 

 sions now employed for oil and gas flames in lighthouses. Such 

 an intensity is about four hundred times that possible at the 

 focus of such apparatus with a flame luminary. Such results as 

 these were probably in the mind of Faraday when he reported 

 that "in principle this light maybe accumulated to any degree." 

 Flashes of the great in'ensity here referred to could only be 

 employed in atmosphere impaired for the transmission of light. 

 In clear weather they would be found to be far too dazzling to 

 the eyes of the mariner, when an intensity of about 50,000 

 candle units is found to be sufficient for his guidance, and in 

 thick fog no possible intensity can be of practical value for 

 navigation. There .ire, however, various gradations of impaired 

 atmosphere between clear weather and thick fog, in which the 

 highest available intensity is doubtless desirable at many import- 

 ant landfall stations for obtaining the greatest possible range of 

 visibility. On the other hand, at the majority of stations in 

 narrow waters, the maximum intensity now obtained with flame 

 light is found to be more generally efficient for navigation than 

 higher intensities. 



In 1881, the question of the relative merits of the three light- 

 house illuminants — electricity, gas, and mineral oil — was receiving 

 the attention of the lighthouse authoiities of this country, which 

 resulted in the Trinity Jiouse accepting the responsibility of 

 carrying out an investigation at the South Foreland, of universal 

 importance to the mariner. In the photometrical and electrical 

 portions of this work, the Trinity Hou<5e were aided by the 

 labours ofProf. Harold Dixon, F.R.S,, and Prof. W.Grylls Adams, 

 F.R.S., which contributed very largely to the success of the 

 investigation. The experiments were carried on during a period 

 of over twelve months, and a vast amount of very valuable 

 evidence was collected from numerous observers, trained and 

 untrained, scientific and practical. The Report of the Com- 

 mittee was presented to both Houses of Parliament, by command 

 of Her Majesty, in 1885. The final conclusions of the Commit- 

 tee are given in the following words : "That, for the ordinary 

 necessities of lighthouse illumination, mineral oil is the most 

 suitable and economical illuminant, and that for salient head- 

 lands, important landfalls, and places where a very powerful 

 light is required, electricity offers the greatest advantages." 



I have already referred to the necessity, with the present deve- 

 lopment of maritime commerce, that every beacon light main- 

 tain a clearly distinctive character. When the optically unaided 

 flames of coal fires were the illuminants of our lighthouses, 

 distinctive characters, owing to the small number of lights then 

 employed, were of little importance, and the only distinctions 

 then possible were the costly ones of single, double, or triple 

 lighthouses at one station ; but, with the enormous inrrpase that 

 has since occurred in the floating commerce of the world, and 

 with the necessary laws now in operation, requiring all vessels 

 to carry lights, trustworthy individuality in coast beacon lights 

 has become a positive necessity. Until very recently, the 

 distinctive chaiacters consisted of the followinir, viz. fixed 

 white, fixed red, revolving white, revolving red, and revolving 

 white and red alternately. The revolving lights showed a flash 

 at periods of ten seconds, twenty seconds, thirty seconds, one 

 minute, two minutes, three minutes, and four minutes. There 

 were also intermittent or occulting lights, having an eclipse at 

 periods of half a minute, one minute, or two minutes. It is 

 now generally considered that fixed lights are no longer trust- 

 worthy coast signals, owing to their liability to confusion with 

 other lights, both ashore and afloat. It is also considered that, 

 in these days of high speed vessels, the period of the character 

 of a coast light should not, if possible, exceed half a minute. 

 The revolving or flashing class of lights are probably the 

 most valuable, on account of their superior intensity as 

 compared with the fixed or occulting class, the light 

 during the intervals of eclipse being condensed into 

 each succeeding flash, by the revolving lenses or reflectors, 

 and thus, with the same expenditure of the illuminant, an 

 intensity is obtained in the flashes of five to eight times 

 that of the fixed or occulting clas«. Where local dangers 

 are required to be guarded by coloured sectors of danger light 

 with well defined limits, this can only be accooapli hed with the 

 fixed or occulting class of lights. We will illustrate this with the 

 model before us. \^"e will also show the clear difference of 

 character, not generally realized, between flashing and occulting 

 lights. A system of occulting lights for lighthouses was proposed 



by the late Charles Babbage, F.R.S., in 185 1, but as it excluded 

 the flashing or most powerful of the existing lights, it did not 

 receive much favour from lighthouse authorities. In 1872, 

 distinctive characters for coast lights was the subject of a paper 

 by Sir William Thomson, F. R. .S., at the Brighton meeting of 

 the British Association for the Advancement of Science, when 

 he directed attention to the extreme importance of ready 

 identification of lights at sea, and proposed the use of quick- 

 flashing lights, their flashes being of longer or shorter duration ; 

 the short and long flashes representing the dot and dash of the 

 Morse alphabet as used in telegraphy. It was found, however, 

 that the number of symbols in our alphabetical code would not 

 be sufficient, on a thickly lighted coast, to insure individuality, 

 and render each distinction perfectly trustworthy. Further, that 

 very rapid repetition of each symbol is not required by the 

 mariner, and would involve loss of accumulative power in the 

 flashes, besides incurring unnecessary wear and tear in rotating 

 heavy optical apparatus. Yet much is to be done in the direc- 

 tion of simple distinction. At the Montreal meeting of the 

 British Association, in 1884, I submitted a paper on " Improve- 

 ments in Coast Signals, "in which were suggested, two alphabetical 

 codes of flashing lights, and one of occulting, all having the 

 same period of the symbol, viz. half a minute. In one of the 

 codes of flashing lights, long and short flashes were proposed, 

 as previously by Sir William Thomson ; and in the other there 

 were proposed white and red flashes. In the occulting series, 

 long and short eclipses were proposed to be substituted for the 

 long and short, or white and red, flashes, of the flashing codes. 

 The system has the advantage of application to all existing 

 lighthouse apparatus, and many lights have been altered to 

 selected symbols of each of these series. 



Little was ever accomplished in the way of warning or guidance 

 to the mariner, during fog, until about the middle of this century. 

 Previously, a few bells had been established at lighthouses in this 

 country and abroad, and gongs of Chinese manufacture had been in 

 general use on board our light- vessels, but both instrumentsarenow 

 acknowledged to be wanting in the efficiency now demanded in 

 fog, to meet the requirements of navigation. The first important 

 improvement in fog signals for the service of mariners was made 

 by the late Mr. DaboU in 1851, who submitted to the United 

 States Lighthouse Board, in that year, a powerful trumpet, 

 sounded by air compressed by horse-power. The apparatus 

 was in-tailed at Beaver Tail Point, Rhode Island, and the 

 favourable results obtained with it stimulated Mr. Daboll, 

 under the encouragement of the United States lighthouse 

 authorities, to the further development of thei apparatus ; and 

 ultimately he employed Ericsson's caloric engine as the motive 

 power, with automatic gearing for regulating the blasts. In 

 1854, some experiments on different means of producing sounds 

 for coast signals were made by the engineers of the French 

 Lighthouse Department, and in 1861-62 MM. Le Gros and 

 Saint- Ange Allard, of the Corps des Ponts et Chaussees, conducted 

 a series of experiments upon the sound of bells, and the various 

 methods of striking them. In 1862, Mr. Daboll submitted his im- 

 proved fog trumpet apparatus, of about three horse-power in the 

 blasts, to the Trinity House, who, under the advice of Faraday, 

 made experimental trials with it in London, and afterwards gave 

 it a practical trial at the Dungeness Lighthouse, where experi- 

 ments were made with it, against bells, guns, and a reed fog- 

 horn of Prof. Holmes, whose services have been already 

 referred to in connection with the first practical application 

 of the electric light. This fog-horn of Holmes was sounded 

 by steam, direct from one of the boilers employed at 

 the station for his electric light. The results of these experiments 

 were in favour of Daboll's trumpet ; and in 1869, one of these 

 instruments was installed on board the Ncwarp light-vessel. 

 In the same year. Holmes having effected further improvements 

 with his steam horn, his apparatus was fitted on board two 

 light- vessels, and sent out to the c -astof China, where they were 

 found to give great satisfaction, as compared with gong signals. 

 In 1863, a Committee of the British Association for the Advance- 

 mtnt of Science memorialized the President of the Board of 

 Trade, with the view of inducing him to institute a series of 

 experiments upon fog signals. The memorial, after briefly setting 

 forth a statement of the nature and importance of the subject, 

 described what was 1 then known respecting it, and several 

 suggestions were made relative to the nature of the experiments- 

 recommended. The proposal does not appear to have been 

 favourably entertained by the authorities to whom it was referred, 

 and the experiments were not carried out. In 1864, a series of 



