27:3 



NA TURE 



vy^' 



1886 



electrical tower as 100 and in the other column the esti- 

 mated brilliancy of the lights exhibited by the other two 

 towers as compared with it. It seems probable that the 

 recorded numerical values of the relative brilliancy of the 

 lights can only be a very rough approximation, and that 

 the figures can hardly be taken as indicating with any 

 degree of precision how much brighter one or other of 

 the lights was on any particular occasion. This would 

 probably be admitted by all who have any acquaintance 

 with actual photometric measurements, and who there- 

 fore know how difficult it is to form any reliable judg- 

 ment of the relative illumination of two surfaces, even 

 when these surfaces are actually in contact, excepting the 

 relation of equality. In the case of the experimental hghts 

 the comparison must have been rendered still more difficult 

 by the fact that what was to be compared was not the com- 

 parative illumination of two moderately bright surfaces in 

 close proximity, but the comparative brilliancy of two 

 lights at some distance from each other, their very bright- 

 ness adding to the difficulty. 



Still these estimations are manifestly valuable as setting 

 forth in a clear and unmistakable form that, to the 

 average observer, a particular light appeared the most 

 brilliant ; and such seems to have been the way in which 

 they were regarded by the Committee, for on page 21 

 they state " it will be evident that by mere eye-measure- 

 ment proportions can only be approximately determined, 

 although the order of superiority may be accepted as 

 proved." 



The results of these determinations are set forth in 

 four tables, from which it appears that in clear weather, 

 and in weather that, although not absolutely clear, was 

 not very foggy, there was no question as to the absolute 

 superiority of the electric light over both its competitors, 

 the electric light in the single form hiving a superiority 

 of more than 30 per cent, assigned to it, as over gas, or 

 oil, in their highest powers {i.e. quadriform for gas, and 

 triform for oil) ; the large-sized gas-burner, with 108 jets, 

 appears to have been slightly superior to the six-wick oil 

 burner, and consequently the quadriform gas to the tri- 

 form oil. 



The eye-observations of the second kind, those in 

 which the distances at which the lights were visible in 

 foggy weather were recorded, gave much the same 

 result : that the electric light penetrated through the fog 

 to the greatest distance, and that the oil and the g:s 

 were about equal in their penetrating power. 



These observations also showed that in the case of the 

 electricity the best result was obtained when the currents 

 produced by two or even three machines were sent 

 through a single lamp, and not when each of the lamps 

 was worked by its own special current. 



The photoaietric measurements were carried out by 

 Mr. Dixon, Mr. Harcourt's pentane flame being used as 

 the standard. As is well known, Mr. Harcourt's standard 

 is an air-gas flame which, unlike the so-called standard 

 candles still commonly used for photometric purposes, is 

 not subject to irregular variations in its light producing 

 powers. Part 11. of the Report contains a full account 

 of the standard flame, and the two arrangements for pro- 

 ducing it, both of which were in use at the South Fore- 

 land. In Mr. Harcourt's original arrangement the air- 

 gas was made and stored in a gas-holder by causing a 

 volume of pentane to diftuse into a known volume of air, 

 and then burning the mixture under certain definite con- 

 ditions which could be accurately produced at all times. 

 The conditions were such that the flame emitted the same 

 amount of light as an average sperm candle burning 

 under the conditions laid down in the Acts of Parlia- 

 ment which control the quality of the metropolitan gas 

 supply, an amount of light which may differ considerably 

 from that emitted by any single candle. 



Mr. Harcourt's pentane lamp was also used : in this 

 arrangement the air-gas is produced as it is required. 



The lamp is very simple in construction, and the flame is 

 just as constant as in the older form, and as easily regu- 

 l.ited, whilst, unlike the older form, the lamp is extremely 

 portable, the whole apparatus not occupying much more 

 space than a packet of candles. 



Two kinds of photometer were used : a bar-photometer 

 with a Leeson star disk, and Mr. Harcourt's table-photo- 

 meter. The latter is a variety of shadow-photometer, 

 and possesses two special advantages: — (i) In common 

 with all shadow-photometers the two sources of light are 

 on the same side of the illuminated surface, and therefore 

 there is less risk of the results being rendered untrust- 

 worthy by diffused or accidentally reflected light than 

 when, as in the more commonly employed arrangements, 

 the sources of light are on opposite sides. (2) The com- 

 parison being made by altering the size of the flames, and 

 not their distance, the two portions of the illuminated surface 

 do not alter their relative position, and are always in that 

 which is most favourable for comparison, accurate juxta- 

 position. The difference in colour between the arc light 

 and the pentane rendered it impossible to employ the 

 shadow-photometer for the estimation of the electric 

 light. For these measurements a Leeson star disk was 

 employed, and it was found that reliable measurements 

 could be obtained by placing the disk between the two 

 lights and moving it to and fro until the pattern of the star 

 was equally distinct on either side, although on the two 

 sides the colours of the pattern and the background were 

 reversed. 



There was so little difference between the colour of the 

 gas and oil flames and that of the pentane flame, that in 

 the case of these two illuminants measurements could be 

 made both with the star disk and with the shadow- 

 photometer. 



Comparisons were made in the photometric shed of 

 the light emitted by the De Meritens electric lamp ; the 

 Wigham gas-burners with different numbers of jets up to 

 the maximum of 108 ; the Douglass .A.rgand gas-burner ; 

 the Sugg gas-burner ; and the Siemens regenerative gas- 

 burner ; and also the six- and seven-wick Douglass oil- 

 burners. 



The amount of light emitted by each of the experi- 

 mental lighthouses was also determined, the observations 

 being made in the huts which had been erected for this 

 purpose at difierent distances from the towers. At the 

 hut nearest to the towers the light from all the burners 

 could be compared directly with the pentane lamp giving 

 the light of one candle, but at the second hut only the 

 electric light and the higher powers of the gas and oil 

 lights could be directly compared with the pentnne lamp; 

 the single gas and oil lights had to be condensed by a 

 lens before accurate measurements of them could be 

 taken ; an achromatic lens, lent by the Astronomer- Royal, 

 was used for this purpose. The fraction of the light lost 

 by the absorption and reflection of the lens was experi- 

 mentally determined and allowed for in the observation. 



The general result of a very large number of observa- 

 tions appears to have been that there is but little to 

 choose between oil and gas as far as their illuminating 

 powers are concerned, and that ele.tricity is greatly 

 superior to both. 



The experiments brought out one fact of great 

 practical as well as scientific interest — that remarkable 

 changes in the transparency of the air occur without any 

 visible haze or mist. To quote Mr. Dixon's words : — 

 " Invisible clouds seemed to float by, obscuring the 

 lights for a time as they passed across our line of vision. 

 Sometimes the French lights at Calais and Cape Grisnez 

 showed brilliantly, when the photometer at Hut 2 proved 

 that the lights from the experimental towers, only a mile 

 and a quarter away, had lost one-fourth to one-third their 

 power." 



With a view of further investigating the fog-penetrating 

 powers of these different lights, the photometer shed was 



