A Treatise On The Principles And Practice Of Harbour Engineering

204 HARBOUR ENGINEERING. The lighthouse is a tall structure, occasionally of wood, but rauch more commonly of stone or iron, rising oftentimes to a considerable height above the water level. When, however, a natural headland or cliff lends itself to the purpose, the structure is not necessarily lofty, and, indeed, for channel lighting, no great height is essential. The building is usually planned in a series of stages or floors, the lantern containing the illuminating apparatus being located at the suminit. The practice of channel lighting, developing through luminous buoys to red sand lighthouse. lightships, attains its highest degree of utility and Fie. 230. perfection in the lighthouse. Sources of illumina- tion for lighthouse use are not only numerically and potentially greater than those available for buoyage service, but they are also of a much more diverse nature, including electricity, coal gas, mineral, vegetable and animal oils, oil gas, and acetylene. The feeble and ineffective candie, which maintained its footing for at least thirty years of the last century, has now entirely dis- appeared. Its great modern prototype is the electric arc, the crater of which possesses an in- trinsic radiance of over 55,000 candle-power per square inch of illuminating surface. Great and remarkable, indeed, have been the strides of late years in the development of lighthouses. Beams of light can now be projected far beyond the limits of their geographical range. The mariner sees their reflection in the sky before he cornes within direct visual contact with them. This, of course, applies to landfall lights, and not to the class of lights which form the subjective basis of this chapter. Channel lighting is achieved perfectly satisfactorily with the aid of lights of a far lower calibre. The electric light is rarely, if ever, employed for this purpose. For general use, the incandescent petroleum vapour burner is more convenient and much less costly, and its light is sufficiently powerful for all stations other than those of primary and special importance. The system has, indeed, only been introduced into this country since the beginning of the present century, but it was adopted in the French lighthouse service several years previously, and it may now be said to have attained general récognition. Prior to this, the wick burner was so prevalent as to be practically universal, either flat, as in the earlier instances, or cylindrical on the Argand principle, with as many as six, eight, or ten wicks arranged in concentric rings. Despite the inferiority of the wick apparatus to the electric arc, its illuminative power was a very long way ahead of the feeble glimmer emitted by the cluster of tallow candles which lit up the summit of Eddystone a century ago.