Orders of the French Lights. The dioptric lights used in France are divided into four orders, in relation to their power and range; but in regard to their characteristic appearances, this division does not apply, as, in each of the orders, lights of identically the same character may be found, differing only in the distance at which they can be seen, and in the expense of their maintenance. The four orders may be briefly described as follows:—
1st, Lights of the first order having an interior radius or focal distance of 36·22 inches (92cm.), and lighted by a lamp of four concentric wicks, consuming 570 gallons of oil per annum.
2d, Lights of the second order having an interior radius of 27·55 inches (70cm.), lighted by a lamp of three concentric wicks, consuming 384 gallons of oil per annum.
3d, Lights of the third order, lighted by a lamp of two concentric wicks, consuming 183 gallons of oil per annum. The instruments used in those lights are of two kinds, one having a focal distance of 19·68 inches (50cm.), and the other of 9·84 inches (25cm.).
4th, Lights of the fourth order, or harbour-lights, having an internal radius of 5·9 inches (15cm.), and lighted by a lamp of one wick, or Argand burner, consuming 48 gallons of oil per annum. This apparatus is, as already noticed, now more generally used of a larger scale, having a focal distance of 9·84 inches (25cm.), and a lamp of two concentric wicks, consuming about 130 gallons of oil per annum.[73]
[73] An apparatus of 0m.·185cm. was recently added to the list of French lights under the name of the Fifth order; while that of 25cm. radius has been called the Fourth, and that of 15cm. radius is styled the Sixth order. Those minute subdivisions I consider to be unnecessary.
Distinctions of the Dioptric Lights. Those four orders are not intended as distinctions; but are characteristic of the power and range of lights, which render them suitable for different localities on the coast, according to the distance at which they can be seen. This division, therefore, is analogous to that which separates our lights into sea-lights, secondary lights, and harbour-lights, terms which are used to designate the power and position, and not the appearance of the lights to which they are applied.
Each of the above orders is susceptible of certain combinations, which produce various appearances, and constitute the distinctions used for dioptric lights; but the following are those which have been actually employed as the most useful in practice:—
The first order contains, 1st, Lights producing, once in every minute, a great flash, preceded by a smaller one, by the revolution of eight great lenses and eight smaller ones combined with eight mirrors; 2d, Lights flashing once in every half minute, and composed of sixteen half lenses. Those lights may have the subsidiary parts simply catoptric, or diacatoptric; and, 3d, Fixed lights, composed of a combination of cylindric pieces, with curved mirrors or catadioptric zones ranged in tiers above and below them.
The second order comprises revolving lights with sixteen or twelve lenses, which make flashes every half minute; and fixed lights varied by flashes once in every four minutes, an effect which, as already noticed, is produced by the revolution of exterior cylindrical pieces.