Sept. 23, 1886 | 
hood has no hedges and few trees, and affords facilities for 
oservations at distances of between 2 and 3 miles. The 
station is provided with surplus steam power for driving experi- 
mental machines for electric lights, and it is easily accessible 
from London. 
Three rough timber towers of sufficient strength to withstand, 
without tremor, the effects of heavy gales were erected at the 
rear of the High Lighthouse, 150 feet apart. These towers 
were marked in large letters, A, B, and C. A tower was 
devoted to electricity, B to the gas system of Mr. Wigham, and 
C to such gas or oil lamps as might be proposed to, and ap- 
proved by, the Committee for trial during the experiments. A 
lantern of the usual first-order dimensions, but with an addi- 
tional height in the glazing for the passage of beams from super- 
posed optical apparatus of the first order, was provided for each 
tower. The optical apparatus in each lantern was, in the outset, 
special in relation to the illuminant to be used for producing 
fixed and flashing lights. For the electric arc lights, optical 
apparatus of the second order of Fresnel was adopted, the 
apparatus having a focal distance of 700 mm. ‘The dimensions 
of this apparatus are greater than optically required for the 
largest electric arc light yet tried for lighthouse illumination, but 
the internal capacity is found to be only just sufficient for the 
perfect manipulation of the light by a light-keeper of possibly 
robust build. For the large gas and oil flames in the A and C 
lanterns the apparatus adopted was of the usual first-order size, 
having a focal distance of 920 mm. 
The lanterns were partially glazed on opposite sides, north 
and south, the southern arc being chiefly for observation from 
the sea. To the northward the land is better adapted for obser- 
vations on shore, and here three observing-huts were erected 
at the respective distances of 2144, 6200, and 12,973 feet ; each 
hut was provided with accommodation for two watchers, and a 
chamber fitted with a large plate-glass window in the direction 
of the experimental lights, and special apparatus for their 
photometric measurement. The third hut proved to be practic- 
ally of but little value for photometry, the distance being too 
great ; it, however, afforded an accurately known distance for 
eye-measurements, and a barrack and starting-point for watchers 
endeavouring to determine the vanishing distance of each light 
during hazy weather. In this they were further assisted by white 
painted posts, placed throughout the whole track to the experi- 
mental lighthouses, at distances of 100 feet apart, the distance 
of each post from the lights being plainly marked on it in black 
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 con- 
venient position, 380 feet long by $ 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 ina 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 arrange- 
ments were made for triple electric luminaries, and experiments 
were carried out with these at comparatively low intensities, it 
was soon found that all the electromotive force available at the 
station could be conveniently applied with efficiency and per- ' 
manency in one compact focal luminary, and its optical apparatus. 
Chis fact demonstrated that the electric arc has the most import- 
ant requisites of a lighthouse luminary ; viz. maximum intensity 
and minimum focal dimensions, and in all states of the atmo- 
sphere, from clear weather to thick fog, an incontestable superior- 
ity over the utmost accumulative efforts of its rivals—gas and 
oil. It was therefore considered to be unnecessary to incur addi- 
uonal 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 intens- 
NATURE 
595 
ity 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 difference was found to effect no appre- 
ciable 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 ad- 
vantage. 
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 mea- 
surements of the three illuminants showed (r) 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 all 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 intensity at disposal that a small percentage of pene- 
trating efficiency is of no practical importance. 
In 1836 Faraday showed by actual experiment that the pene- 
trating power of alight in atmosphere impaired by such obstruc- 
tion as fog, mist, &c., is but very slightly augmented by a very 
considerable increase in the intensity, and M. Allard, late 
Engineer-in-Chief to the French Lighthouse Board, has more 
recently shown after long experimental and practical research, 
that, in an atmosphere of average transparency, a beam of light 
equal to 6250 becs (Carcel) would penetrate 53 kilometres, yet 
when augmented to twenty times that intensity, or 125,000 becs 
(Carcel), it would only penetrate 75°40 kilometres ; showing that, 
in the average condition of atmospheric transparency, 2009 per 
cent. of increased intensity only gives 42 per cent. longer range. 
The South Foreland experiments have demonstrated that, 
while with both gas and oil an ordinary intensity of light can be 
adopted for clear weather sufficient to reach the sea horizon with 
efficiency for the mariner, a maximum light can be shown with 
impaired atmosphere fifteen to twenty times this intensity, and 
that in these respects both illuminants are practically on an 
equality. This maximum light of gas and oil is considered by 
the Committee to be sufficient for all the ordinary purposes of 
navigation, and, for this, mineral oil is the most economical 
illuminant ; but for some special cases, where the utmost inten- 
sity and penetration are demanded, these results can only be 
attained by electricity, and by this agent an intensity more than 
ten times that of the maximum of either oil or gas is found to be 
practically available. 
With regard to the gas and oil lights, the report of the Com- 
mittee states that ‘It appears from the direct eye-observations, 
made at distances varying from 3 to 27 miles in clear weather, 
that through annular lenses, light for light, there is practically 
no difference. Both reach the horizon with equal effect. In 
weather not clear the records indicate practically the same 
relation. In actual fog, again, the records indicate a general 
equality of the lights. Both are lost at the same time, both 
are picked up together ; and although here and there a very slight 
superiority is attributed to the gas, this superiority is of no value 
whatever for the purposes of the mariner,” A point referred to 
in favour of gas is the well-known one of greater handiness and 
ease of manipulation than oil, which is of importance for small 
beacon lights, where a constant attendant is not provided ; but 
this does not apply to a coast light, where a light-keeper is 
always required to be on the watch in the lantern from sunset to 
sunrise. \Vith oil the great advantage, in addition to economy, 
lies in the simplicity of its application to a coast lighthouse in 
