TRANSACTIONS OF SECTION G. 797 



each light during hazy weather. la this they were further assisted by white 

 painted posts, placed throughout the whole track to the experimental lighthouses, 

 at distances of 100 feet apart, the distance of each post from the. lights heincr 

 plainly marked on it in Mack figures. For the more exact examination and 

 measurements of the intensity of each luminary and that of the beam from each 

 optical apparatus, a photometric gallery was erected in a convenient position, 380 

 feet long by S feet wide, and provided with all the necessary appliances. 



During a period of over twelve months the experimental lights were exhibited, 

 and watched by numerous observers, trained and untrained, scientific and 

 practical. During that period a vast amount of valuable evidence was collected ; 

 by the aid of which the Committee were subsequently enabled to state their con- 

 clusions with definiteness. During these investigations intensities were shown in 

 a single oil and gas luminary about three times greater than the electric arc 

 luminary first adopted at Dungeness in 1861, while, with a single electric arc 

 luminary, there was shown a practically available focal intensity about fifteen 

 times greater than that of the Dungeness luminary, and the highest yet shown to 

 be practically available for the service of the mariner. 



With gas and oil the highest intensity of a single luminary and optical 

 apparatus was tripled by the use of three superposed luminaries and optical 

 apparatus, and although optical arrangements were made for triple electric lumi- 

 naries, and experiments were carried out with these at comparatively low in- 

 tensities, it was soon found that all the electro-motive force available at the station 

 could be conveniently applied with efficiency and permanency in one compact focal 

 luminary, and its optical apparatus. This fact demonstrated that the electric arc 

 has the most important requisites of a lighthouse luminary ; viz., maximum 

 intensity and minimum focal dimensions, and in all states of the atmosphere, from 

 clear weather to thick fog, an incontestable superiority over the utmost accumu- 

 lative efforts of its rivals — gas and oil. It was therefore considered to be un- 

 necessary to incur additional cost for exhibiting the electric arc light, under the 

 same conditions of accumulative powers as its rivals, for showing a maximum 

 intensity. "With the best gas and oil luminaries it was found that, where gas of 

 the ordinary commercial quality is employed, there is no appreciable difference, 

 either in the intensity or focal compactness of the luminary, but when the richest 

 gas, from cannel coal, and mineral oil are used, there is found to be a superiority in 

 the maximum intensity of this luminary over oil of about 45 per cent., and in focal 

 compactness of about 10 per cent. ; but in haze and fog, when the maximum 

 intensity only is required, this diflerence was found to effect no appreciable gain in 

 penetrative power, therefore the question of merit between these illuminants was 

 found to resolve itself into one of economy only, and in this respect mineral oil at 

 the present market prices was found to have a considerable advantage. 



The relative penetrability per unit of light of the best gas and oil flames in haze 

 and fog is so nearly identical that the question is of no practical importance in 

 lighthouse illumination. But, with regard to the relative atmospheric absorption 

 of these lights and the electric arc light in certain impaired conditions of the atmo- 

 sphere, the electric arc light is found to compare somewhat unfavourably. The 

 general result of the photometric measurements of the three illuminants showed 

 (1) that the oil and gas lights, when shown through similar lenses, were equally 

 affected by atmospheric variation; (2) that the electric light is absorbed more 

 largely by haze and fog than either the oil or the gas light ; and (3) that aU three 

 are nearly equally affected by rain. Experiments, made in the photometric gallery 

 at the South Foreland with the electric arc light, have shown that the loss by 

 atmospheric absorption is by no means so great as was previously supposed. It 

 would have been most interesting and instructive to have obtained data for exactly 

 determining the relative coefficients of atmospheric absorption of the electric arc, 

 gas, and oil luminaries, but the necessary observations and measurements for 

 effecting this would have prolonged the time too much, and added too much to the 

 cost of the investigation, especially when it is remembered that with the electric 

 arc light there is for coast illumination such an enormous preponderance of initial 



