Since the completion of this lighthouse, Mr. Gordon has been employed by the Ordnance Office to furnish designs and specifications for a tower on the same principle, but of larger dimensions and improved details, which is to be erected on Gibbs’ Hill, in the island of Bermuda.
Chapter IX.
THE LIGHTHOUSE SYSTEM.
Imperfect Illumination of the old Lighthouses—First Improvements—The Argand Lamp and Reflecting Mirrors—Revolving Lights—The Catoptric System—Varieties of Lights—The Dioptric System—Its Details—Introduction of this Method into Great Britain—Comparison of the two Methods—The Drummond and Voltaic Lights—Gurney’s Lamp—Captain Basil Hall’s Experiments—Ventilation of Lighthouses.
Since there is something more or less common in the modes of lighting and in the general economy of all lighthouses, a general view of the subject is likely to prove of more interest than particular details.
In consequence of the rotundity of the earth, the distance at which a beacon light ceases to be visible depends upon its elevation. The height to which a lighthouse may be carried is a simple question of expense. The greater part of the pharos of the Romans were much higher than the most celebrated modern towers. Yet, as it respects optical effect, the feeble rays which were diffused from the wood or coal-fires at their summits, could never have traversed the thick fogs which in all climates occasionally overspread the lower regions of the atmosphere.
Nevertheless, as to the strength of the light, the modern lighthouses were, until lately, little superior to the ancient. At the time of the erection of the Eddystone lighthouse civil engineering was greatly in advance of practical optics. That noble structure was lighted by tallow candles, without reflectors or the aid of any kind of apparatus for concentrating the light. ‘For more than half a century this feeble light was all that directed the mariner in the very high-road of commerce.’ So late as the year 1811 it was lighted with twenty-four wax candles. In 1812 the Lizard lighthouse, certainly one of the most important in the kingdom, was maintained with coal-fires. The Bidstone, a leading light to the port of Liverpool, was furnished with an enormous spout lamp, with a wick twelve inches in width, the smoke from which was so great as to completely darken the upper surface of its reflector. The first important improvement was the introduction of that admirable invention the Argand Lamp, with a double stream of air. Four or five of these lamps would doubtless give as much light as the large fires kept by the Romans; but if those lamps are furnished with reflecting mirrors, the luminous effect is prodigiously increased.
The light of inflamed bodies spreads itself equally in all directions. One portion is absorbed by the ground, another is dissipated in space. The navigator, whose route we are anxious to enlighten, profits only by the rays that proceed in a horizontal direction, or nearly so, from the lamp to the sea. But such of the horizontal rays as are directed towards the land are of course entirely lost to the purposes of the lighthouse. This zone of horizontal rays forms not only a very small portion of the total light, but has also the serious inconvenience of becoming much weaker by divergence, so as to convey to a distance but a very feeble light. To destroy this divergence, and to profit by all the light of the lamp, was the task to be accomplished, before lighthouses could be rendered useful to the full extent.
The application to this purpose of deep metallic mirrors, known under the name of parabolic mirrors, has been found effectual to the purposes required. When a lamp is placed in the focus of such a mirror, all the rays which emanate from it are reflected from the polished surface, and converge in one direction: their original divergence is destroyed, and they form, as they issue from the apparatus, a cylinder of light, parallel with the axis of the mirror. This light would be transmitted with undiminished brilliancy to a great distance, did not the atmosphere absorb a portion of it.