Let us draw near, however, and carefully examine the lamp now before us, because in several details it differs from those we have described. It will specially interest us as an English invention.
At the epoch when Teulère and Argand had made the progress already specified in the construction of a suitable lamp, Rumford, desirous of effecting a still greater improvement, asked himself whether, by adapting it to burners with several concentric wicks, it would not be possible to increase its power of illumination. The attempt was made, but did not prove successful; he experienced considerable difficulty in regulating the flame of these multiple wicks, and in preventing their carbonization under the action of the intense heat developed by their combination. It was the study of this question which led Fresnel and Arago to their beautiful experiments on the illumination of lighthouses.
After repeated essays, these two men of science decided on the type of the lamp which we are now contemplating; an instrument remarkable not only for the whiteness and intensity of its light, but also for what I may call its power of endurance; it will burn for upwards of twelve hours without requiring to be touched. And that this advantage is most important the reader will apprehend, when he remembers that the lighthouse-flame must be kept kindled throughout the longest nights of winter.
At the present day, lighthouses of the third class are illuminated by lamps with two concentric wicks; which, in a certain sense, means two lamps in one. In lighthouses of the second class, each lamp has three; and in those of the first class, each has four wicks. In the latter we obtain, with a single illuminating apparatus, the full power of twenty-three Carcel lamps. The luminous focus, though gifted with so much potency, presents, nevertheless, but a flame of moderate breadth, and its light is as white as it is brilliant.
The oil employed in the lighthouses of Great Britain, Ireland, and France is the colza, which has of late years entirely superseded spermaceti oil, as producing an equal quantity of light at little more than half the expense. The electric light has, however, been proposed as a more powerful method of illumination. One system, in which the light is produced between carbon points by the revolution of magnets fixed on wheels, worked by a steam-engine, was tried with much success by Professor Holmes at the South Foreland; and is still, we believe, in use at Dungeness, as it is, in France, at the two lighthouses of La Hève. In the latter case, the mechanism producing the currents is composed of two steam-engines, each with a five-horse power, and of four electro-magnetic machines of six discs, composed each of sixteen bobbins. It is placed in a boat, adapted for the purpose, at an equal distance from the two towers. Under ordinary atmospheric conditions, a single steam-engine is kept in motion, communicating with a magneto-electric machine for each lighthouse. During fogs and mists, both engines are in activity, and each lighthouse receives the currents of two magneto-electric batteries, which are then associated.