44 



NA TURE 



[Nov. 



1886 



submitted to the Committee, and hoping that some of them 

 may be of service. (Signed) 



July 26, 1884 A. Vernon Harcourt 



Since' the foregoing preliminary report was presented to the 

 Board of Trade, the experimental inquiry has come to an end, 

 and a complete account of the apparatus, observations, and test- 

 ings has been published by the (jomuiittee of the Trinity House 

 who had charge of the inquiry, followed by a statement of the 

 conclusions at which the Committee have arrived. 



I propose to arrange the remarks I have to offer under the 

 following heads : — 



I. — Apparatus for the exhibition of the experimental lights. 

 II. — Arrangements for observation. 

 III. — Photometry. 

 IV. — Comparison of lights. 



V. — Range of lights in hazy weather. 

 VI. — Cost of each system. 



I. — Apparatus /or the Exhibition 0/ Experimental Lights 

 In my preliminary report I have given a general description of 

 the temporary towers, the lenses and lamps. In Parts I. and II. 

 of the Trinity House Report are to be found plans and measure- 

 ments giving the full details of these constructions. The towers 

 are admirably suited to their purpose, and their situation 

 and the distance between them proved most convenient for 

 observation. 



In regard to the arrangements for exhibiting the electric light, 

 it is to be observed that, although the electric light completely 

 outshone its competitors, it was heavily handicapped in the 

 competition. The "leads" were not of sufficient calibre to 

 carry the large electrical currents used, for a distance of nearly 

 300 yards, without considerable loss. Prof. Adams estimates 

 the loss at more than one-fourth the electrical energy supplied. 



The five vertical prisms used in the case of the electric arc 

 to bring together the horizontal rays, subtended an angle of 

 only 30", while the annular lenses which served the same pur- 

 pose in the gas and oil lanterns subtended an angle of 60°. 

 Thus the fraction of the light emitted from the central 

 source of light, which composed the revolving beam, was 

 only half as great in the case of the electric arc, as in the 

 case of the gas and oil flames. It seems probable also that a 

 beam of less divergence may be used with advantage to obtain 

 a maximum range in hazy weather ; and such a beam may be 

 obtained from the electric arc with lenses of moderate size. On 

 a few occasions when an annular lens similar to those used in 

 the other two lanterns was placed in front of the electric arc, the 

 light was dazzling at a distance of more than a mile, and 

 surprisingly vivid at a distance of 20 miles. I see that on 

 a clear night when the loS-jet gas burner behind an annular lens 

 gave a light of 60,000 candles, the electric arc behind its cylindri- 

 cal lens and vertical prisms gave a light of 1,200,000 candles, 

 and behind an annular lens a light of 12,000,000 candles. In 

 the one case the arc was five times as powerful as Mr. Wigham's 

 " quadriform," in the other fifty times as powerful. 



For the sake of uniformity and comparison under similar 

 conditions, only the central belt of the Fresnel apparatus was 

 placed round the electric lamp as round the gas and oil burners. 

 The suppression of the top and bottom prisms, though entailing 

 a loss of 30 per cent, of the light produced, is a necessary 

 sacrifice where large burners developing great heat are placed 

 immediately one over the other. But each of the electric lamps 

 ill tower A might have been surrounded with a complete 

 Fresnel apparatus, adding nearly one-third to their light, 

 without any difficulty or any necessity for separating them 

 more widely. 



Thus, if the principle which has been enunciated had been 

 followed, of doing for each illuminant the best th.at could be 

 done within the limits of the lighthouse lantern, if a triform 

 electric light had been exhibited, with leads of low resistance, 

 with a lens subtending; an angle of 60°, and with top and bottom 

 prisms, the power of this light might have been more than 

 tripled. By also reducing the divergence of the beam, which I 

 think might be done with advantage, a further increase of power 

 could have been gained. This fact should be borne in mind in 

 comparing the results which were obtained with the three 

 illuminants. 



' Further Report of Mr. Vernon H,arcourt to the P.oird of Tr.ide on the 

 Experimental Lights exhibited at the South Foreland. 



In M. Allard's interesting and important " Memoire sur les 

 Phares electriques," 1S80, he gives the results of a trial of three 

 Gramme dynamo-machines and an electro-magnetic machine of 

 the Alliance Company. The former gave for the same horse- 

 power 40 or 45 per cent, more light than the latter. But 

 M. Allard measured only horse-power and light, not the 

 electrical energy developed ; and it does not appear whether the 

 larger yield of light was due to a more powerful electrical 

 current, or to the position of the carbons, and the form of the 

 incandescent end-, being more favourable to the emission of 

 light with the continuous current. Probably the De Meritens 

 machines, which produced a light of about 1000 candles per 

 horse-power, are superior to those of the Alliance Company, 

 which yielded only 540 candles, and are equal to the Gramme 

 machines which yielded Soo candles per horse-power. Of all 

 that rel.ates to the economical production of powerful arc lights, 

 knowledge is advancing rapidly. The ample provision of steam 

 power, and the excellent photometric gallery at the South Fore- 

 land, will no doubt be used from time to time for the trial of 

 new types of electrical machines, of regulators, and of carbons. 

 For the past experiment, and apart from the question of cost, 

 the De Meritens machines worked admirably, converting, ac- 

 cording to the measurements of Prof. Adams, mechanical into 

 electrical energy with a loss of only 16 per cent. The current 

 supplied was more than sufficient for the largest carbons ; 

 indeed, carbons exceeding I5 inches in diameter were heated to 

 redness through their entire length. 



In regard to the apparatus for exhibiting the gas system of 

 Mr. Wigham and the oil lamps of the Trinity House, little can 

 be added to the full and clear account of the Trinity House 

 Committee. But as it has been stated, since the publication of 

 the Trinity House Report, that Mr. Wigham's foreman was not 

 left unfettered to make the best dispLiy which the apparatus in 

 his charge would allow, I may here put on record wfiat I saw 

 and believe in the matter. I paid many visits to the gas light- 

 house by d.ay and by night, .and was in frequent communication 

 with the foreman, Mr. Higginbotham, from the beginning to the 

 close of the experiments. The arrangement of each night's 

 programme rested with the Committee of the Trinity House, 

 who so ordered matters that abundant opportunity was given for 

 the observation and measurement of all the varieties of each 

 illuminant. Among these were Mr. Wigham's combinations of 

 28 jets, 48 jets, 68 jets, SS jets, and 108 jets, the re.ady coii- 

 version of one of which into another is among the merits of his 

 ingeniously constructed burner. When the effect of the smaller 

 number of jets was to be observed, it is clear that the full power 

 of the burner could not also be shown. Therefore, there were 

 necessarily times when Mr. Wigham's forem<in was not free to 

 make the best display which the apparatus in his charge would 

 allow. With this exception only, I believe that Mr. Wigham's 

 foreman w.as perfectly free to do his best and make any improve- 

 ments Mr. Wigham or he could devise. I see from the summary 

 in the Trinity House Report that the full power of Mr. Wigham's 

 burners was shown on 127 nights ; and it appears from the photo- 

 metric record that it was measured 57 times. This ought to 

 suffice for an accurate judgment of its merits. 



Comparing the gas and oil towers as they appeared to a visitor 

 when in full operation, the gas had one striking advant.age, and 

 one equally obvious disad^'anta^e. The advantage was that it 

 needed no care. When the lenses had not to be revolved by 

 hand, nor the number of jets changed, one attendant in the 

 tower was sufficient, and he had little or nothing to do. In the 

 oil tower, on the other hand, I have seen a keeper on every one 

 of the three stages, each man watching and from time to time 

 adjusting his lamp. The disadvant.age encountered in the gas 

 tower was the excessive heat from the large gas-burners, which 

 by causing unequal expansion of the glass lenses and their metal 

 framing, and of the outer and inner surfaces of the lenses them- 

 selves, caused cracks to appear, which in the continuous belt of 

 thick glass gradually spread from side to side. But though the 

 burning of gas yields for the same li^ht more heat than the 

 burning of oil, there is no reason to think that with a diminished 

 consumption of gas, e.g. the 160 cubic feet an hour of the 68 jets 

 instead of the 300 of the 108 jets, such a disaster would recur. 

 When the gas flame is surrounded by a chimney, as in Sir James 

 Douglass's and Mr. Sugg's multiple Argands, the heating of the 

 lenses is greatly diminished. 



When the lights were first exhibited, the behaviour of the oil 

 lamps in C tower w.as a matter of much interest. U^ing gas, 

 Mr. Wigham had succeeded in quadrupling the power of a large 



