The apparatus of which they complained was, in truth, exceedingly defective; its lamps, differing but little from those which the seven foolish virgins suffered to die out, had broad wicks, and if they produced but little light, by way of compensation they emitted an enormous amount of smoke. It was natural, therefore, that men of science should, with a view to improvement, first direct their attention to the lamp. The pioneer in this course of inquiry was Argand, who, about 1748, contrived to secure “a double current of air;” which consists, as any one may see in the first lamp he meets with, of a wick, shaped like a hollow cylinder, enclosed in a glass tube. The heat caused by the combustion of the oil produces a vigorous draught, which leads to an abundant circulation of air both internally and externally; and air is for the lamp, as for man, the plant, and the animal—life!
Various modifications and improvements of Argand’s system have been successively introduced. The glass tube, for instance, by one inventor, was contracted at a short distance above the burner, so as to project more immediately the current of air upon the flame, and stimulate combustion. In his turn, Carcel bethought himself of supplying the wick with a superabundant quantity of oil, so as to avoid the heating of the burner, and to render the flame more regular; he thus succeeded in keeping the lamps burning for a longer period without a replenishment of the wick.
There then remained the reflectors. Curved in the form of a spherical segment, these received but a small portion of the luminous rays, and rarely returned them in the proper direction. Teulère, the engineer-in-chief of the province of Bordeaux, who was to distinguish himself at a later period by the erection of the Cordouan Tower, was ordered to make an examination of both the lamps and the reflectors, and to study the best means of remedying the evils complained of. His studies resulted in a paper of great interest, published in 1783. To concentrate in a single direction a large portion of the rays which were lost on all sides, he proposed the use of mirrors of perfect polish and a better form. By causing these mirrors to revolve around a lamp—that is, by projecting successively towards every point of the horizon the lustre formed by a large portion of the rays thus collected into a single sheaf—he invented at the same time the eclipse.
It was not at Cordouan, nevertheless, that the system was first applied, but at Dieppe, where the celebrated Borda, having studied Teulère’s paper, had a small revolving apparatus of five parabolic reflectors made[18] in 1784. The apparatus of Cordouan, likewise established by Borda, was not placed in the lighthouse tower until after its restoration by Teulère—that is, in 1790.
This method of lightage was obviously a great improvement, and all the maritime powers hastened to adopt it. As the Catoptric System, it was, until within the last few years, exclusively employed on the coast of England. Though less esteemed in France, its use has not been entirely abandoned; and the French still employ catoptric apparatus for “the illumination of narrow channels, or for harbour-lights; to strengthen in a given direction a light whose range is sufficient for the maritime horizon generally; to illuminate lightships; and for service as provisional appliances.”
CATOTROPIC APPARATUS.
In the accompanying design we represent a plan and elevation of a catoptric apparatus, which is composed, as will be seen, of nine reflectors arranged in groups of threes. A small rotatory machine sets the system in motion, and eclipses at greater or shorter intervals are obtained by the varying speed with which it is worked. The range of the apparatus depends partly on its power, and partly on its position.