Having said thus much of the central lens and its concentric rings of glass, a few words become necessary in reference to the lamp which feeds them, as it were, with light. Fresnel’s lamp may be shortly described as containing four concentric burners, which are defended from the excessive heat produced by their own combined flames by a superabundant supply of oil. This oil is pumped up from a cistern below by means of a clockwork movement, and overflows the wicks incessantly. To supply fresh currents of air to each wick with a rapidity sufficient to support the combustion, a very tall chimney-tube is found requisite. And yet the wicks do not carbonize with the extreme speed that might be supposed. It is even found, we are told, that after they have suffered a good deal, the flame does not perceptibly decrease, because the intense heat evolved from its mass encourages the rising of the oil in the cotton. Mr. Stevenson informs us that he has seen the large lamp in the Tour de Corduan burn for seven hours, and yet the wicks were neither snuffed nor raised. In the Scotch lighthouses a full flame is often maintained, with Colza oil, for no less a period than seventeen hours, and yet the lamp is untouched.
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.