Chap. VI.
243
The total height of the building above the base of the tower, is 14G feet, and above thesurface of the rock 1G2 feet.
In 1727, the summit underwent a change; the former lantern having been destroyed, oneof iron was substituted. This was done under the direction of M. Betri, engineer-in-chief at Bordeaux, who contrived a cage of iron, or lantern, formed of four principalpillars, supporting a cupola, finished with a large ball and vane SG feet above theplatform ; the lantern was entirely open, and the smoke could escape on all sides ; theceiling, which was circular, was formed into a hollow cone or funnel, the top ofwhich was bent downwards, about 3 feet; the entire sloping surface of the cone wascovered with tin plates, which became so many reflecting surfaces, and occasioned the lightto he seen from a greater distance.
Reflected light was made use of here for the first time about the year 1780, when
Fig. 257.
SECTION OF THE LIGHTHOUSE.
INI. Borda introduced an Argand lamp in the focus of a parabolic mirror; the reflector wasa sheet of copper plated with silver, with a focal length of about 3 or 4 inches ; the diameterof the outer edge was 21 inches.
The curvature of the reflector was truly parabolic ; the light issued from a mathematicalpoint, and the rays were reflected from a mirror, placed exactly parallel to the axis of the ge-nerating curve; the beam of the projected light was that of a cylinder having a diameterequal to that of the mirror. The light, however, in this form, was nearly useless, and itwas found necessary to give the rays a divergence, that they might extend to a greaterportion of the horizon. To effect this, the burner, which was about an inch in diameter,was made to produce the luminous rays at a small distance from the focus, and instead ofbeing reflected in a mass of cylindrical or parallel rays, they were projected in a conehaving a divergence of about forty degrees.
To obtain a sufficient quantity oflight it was necessary to have a number of parabolic
r 2