NA TURE 



\_Nov. II, iSS6 



the burner from a diameter of about 4 inches with 2S jets, to a 

 maximum of II inches diameter and 108 jets. 



The oil lamps are of the usual Trinity House pattern with six 

 concentric wicks, and are fed with parafHn oil. 



Cannel gas is manufactured and stored at a short distance 

 from the experimental towers, and supplied through a meter to 

 the gas-burners. 



For the observation of the lights, which were first shown in 

 the week beginning March 30, three huts have been erected at 

 different distances along a line perpendicular to the line of the 

 towers, and this line has been marked by posts showing the 

 distance from the central tower. The lighthouse-keepers who 

 are stationed in one or other of these huts are instructed to 

 make hourly observations during the time the lights are ex- 

 hibited, expressing in figures their estimate of the relative 

 brightness of the three lights. When the night is misty the 

 keepers are instructed to patrol the line of posts, and to record 

 the distance at which each light is lost or becomes visible. To 

 avoid prejudice in favour of either an old or a new mode of 

 lighting, the towers have been labelled, and are called A 

 (electricity), B (gas), and C (oil). The huts are numbered. 

 No. I is rather more than 700 yards distant from the central 

 tower, No. 2, about l^ mile, and No. 3, 2^ miles. Steps h.ave 

 also been taken to obtain estimates of the relative brightness of 

 the three lights from observers at gi-eater distances. To secure 

 the identification in each hut of the lights observed when all 

 three are not visible, three tubes have been fixed in each hut 

 directed towards the lights A, B, and C, and labelled ac- 

 cordingly. 



The huts serve also for measurements by various photometric 

 methods of the light sent forth from each lantern. A number 

 of such measurements have already been made, the results of 

 which have been communicated to the Board of Trade by the 

 Trinity House. 



Near the engine-house on the South Foreland a long gallery 

 has been built, in which the light emitted by the various lamps 

 employed or proposed to be employed, can be measured so as to 

 ascertain the value of these lamps independently of the lenses by 

 which, within the lighthouse lanterns, their apparent brightness 

 is variously augmented. 



The experimental inquiry thus instituted will serve : — 



(i) To ascertain the amount of light given by the six-wick 

 and seven-wick oil-lamps, and of other oil-lamps, or modifi- 

 cations of them (if any) which may be proposed for lighthouse 

 service. 



(2) To ascertain similarly the amount of light given by 

 Wioham's gas-burners on different scales (28, 48, 68, &c.) with 

 different rates of consumption, and, if thought well, with 

 different qualities of gas, and to test other gas-burners in like 

 manner. 



(3) To furnish further and trustworthy measurements of the 

 light given by the electric arc with various carbons and with 

 various tensions and quantities of electricity, and to test the 

 efficiency of the De Meritens magneto-electric machines in con- 

 verting mechanical into electrical energy, and whether they 

 work without difliculty or risk of break-down or need of repair or 

 loss of power ; also to test the working of the De Meritens 

 electric lamp, and of other electric lamps, if thought well. 



(4) To furnish additional data for estimating the cost of m.ain- 

 taining any given light for a certain time, say 1000 candles for 

 one hour, by each mode of producing light, and on the various 

 scales suitable to different localities. 



(5) To measure the efficiency of the lenses employed, especially 

 with flames of different sizes in their foci. 



(6) To prove experimentally (if such proof be desired) that 

 2 or 3 or 11 similar lights, when juxtaposed, give twice or thrice 

 or « times as much light as a single light gives. 



(7) To ascertain what light is sufficient to be visible from its 

 horizon on a clear night, and in what ratio on the average of 

 many nights the visibility of a light at great distances increases 

 with its total intensity, or lens area, or proportion of red or of 

 blue rays. 



(8) To test the effect of the variations last named in haze, or 

 mist, or fog, or rain, or snow, that is, when the air is made more 

 or less opaque by particles of liquid or solid water of various 

 sizes suspended in or falling through it. Such testing may be 

 made either photometrically, which is only possible in slight 

 haze and at small distances, or by observations of the distance at 

 which each light is lost or reappears. 



(9) To try the quesrion of the utility of ex-focal light, whether, 



that is, it often happens that the position of a lighthouse may be 

 seen by the illumination of cloud or fog above or around it, when 

 its position would be unknown if equal light from a smaller focus 

 were directed almost wholly towards the mariner, and not alloweil 

 to spread. 



(10) To test further whether in mist or haze sudden flashes of 

 a powerful beam of light ai'e noticeable when an equal light 

 maintained constantly, or waxing and waning gradually, would 

 not be noticed. 



It is likely that other subjects of experimental inquiry may be 

 sxiggested by those experienced in lighthouse illui-nination, or 

 may occur as the experiments proceed. But, taking those aliove 

 enumerated in order, I will attempt to indicate the conclusions 

 which at present appear probable, and to make some suggestions 

 as to points still to be investigated. 



(1) It appears that the six-wick oil lamp behind the annular 

 lens sheds light of as great intensity as the seven-wick lamp, 

 while its consumption of oil is much smaller. ProbaMy this 

 result is due, in part, to the fact that the outer ring of flame 

 which the seventh wick adds is further from the focus of the lens, 

 while each ring of flame is partially opaque to the light from the 

 rings inside it ; and partly to the fact that the seven-wick lamp 

 has not yet been brought "to so perfect an adjustment of oil- 

 supply to air-supply as the six-wick lamp. 



I do not know whether any oil-lamp used in other than 

 English lighthouses is such as to merit a trial against the 

 Trinity House lamp. 



(2) Some observations have been made with Mr. Wigham's 

 burners with 88 and with 108 jets, which seem to show that with 

 gas as with oil, behind the annular lens, no gain in intensity of 

 light results from the circaposition of another ring of flame. 

 Some evenings should, I think, be devoted to trying this 

 question out. The value of ex-local light behind an annular 

 lens seems to be almost nil as regards intensity, and, if so, it may 

 be well to use with revolving light a smaller flame than that of 

 the six-wick lamp. Excellent experiments on this question can 

 be made with Mr. Wigham's burner by exhibiting on a clear 

 night through the annular lens one of these burners, whose size 

 should be reduced, after an interval sufficient for photometry, 

 from loS jets to 88, and so on to the smallest size, measuring 

 also after each change the consumption of gas. It will probably 

 be found that a large fraction of the directed light is still obtained 

 with a relatively small consumption of gas, and with the accom- 

 panying advantage of a low temperature within the lantern. 



Similar measurements should be made with a cylindrical lens 

 and with the naked flame in the photometric shed. 



At present one other gas-burner besides Mr. Wigham's has 

 been tried, a ten-ring gas-burner devised by Sir J. Douglass, 

 which has given an excellent yield of light. Two others, by 

 W. Sugg and Co., and by the F. Siemens Company, await a 

 trial. The problem which the maker has to solve is to pack as 

 much highly luminous flame as possible into a sphere of 3 or 4 

 inches diameter. 



Whei'e gas has to be manufactured expressly for a lighthouse, 

 it would generally be b;st to' make cannel gas, but near a town 

 where common gas could easily be laid on, it would be cheaper 

 to use common gas. It might, therefore, be worth while during 

 the course of the experiments to charge the small gas-holder 

 with common gas, and to note the consumption and the light 

 developed. It would probably be found that with suitable 

 burners the chief disadvantage in using common gas was the 

 greater development of heat, the same light being obtained from 

 the consumption of a larger volume of lower priced gas. 



(3) Many measurements have been made in recent years of the 

 light of the electric arc, but the difficulty of making measure- 

 ments of so variable a light, and the uncertainty attaching to the 

 standards of light employed, and the great differences between 

 one arc light and another, according to the electric current and 

 the carbons employed, make it clearly desirable to have further 

 measurements of the electric light at the South Foreland. 



Photometry should be accompanied, as with oil and g.as, by a 

 measurement of consumption. The mechanical energy absorbed 

 can be measured at the strap which connects the magneto 

 electric machine with the steam-engine. The electrical energy 

 developed can be measured in tension maintained, and quantity 

 used, at the leads connecting the machine with lantern A. The 

 cost of each horse-power per hour on the actual scale of working 

 at the South Foreland must 1 e already known. The rate at 

 which the two forms of carbons which have been tried are con- 

 sumed is also known. 



