The only risk in using the Fresnel lamp, says Mr. Stevenson, arises from the liability to occasional derangement of the leathern valves that force up the oil by means of clockwork. Several lights on the French coast, and, more especially, the Tour de Corduan, have been extinguished by the failure of the lamp for a few minutes; an accident which has never happened, and scarcely can happen, with the fountain lamps of the Catoptric system. To prevent such dangerous mishaps, which, under some circumstances, might entail the loss of a “tall ship,” various precautions have been adopted. The most efficacious seems to be this: an alarum is attached to the lamp, consisting of a small cup pierced in the bottom, which receives a portion of the oil overflowing from the wicks, and is capable, when full, of balancing a weight placed at the opposite end of a lever. The moment the machinery stops, the cup ceases to receive the supply of oil, and the remainder escaping at the bottom, the equilibrium of the lever is destroyed; it falls, and disengages a spring, which rings a bell with sufficient force to arouse a sleeping keeper. But, says Mr. Stevenson, shrewdly, it may justly be doubted whether such an arrangement might not actually tempt a keeper to relax in his vigilance, and rely on the alarum to waken him in case of need. In all the dioptric lamps on the British coast, therefore, the converse method is adopted of causing the bell to cease when the clockwork stops.
Another and more important precaution consists in keeping always at hand, in the light-room, a spare lamp, trimmed, and adjusted to the proper height for the focus, and in every respect ready to act as a substitute for the other if any accident occurs.
But while I am tracing these words, I read that experiments have been successfully made with gas for the illumination of the lenticular apparatus, and that, if it will afford a steadier and fuller light, at less expense, and with no risk of accident, it will probably be adopted.
To continue:—
Once having acquired a full command of all the rays amplified from the lamp, the next desideratum was to diversify the appearance of the light which they constituted; for, as I have already said, it is not enough to stretch a belt of warning fires around the coast,—we must take care that each shall in some wise be distinguished from the other, so as to afford the navigator a clue to its particular locality. Hence arose the division into fixed, revolving, intermittent lights, and so on, which I have already described, and which is secured in the following manner:—
FRESNEL’S REVOLVING LIGHT.