136 



SCIENCE. 



[Vol. I., No. 5. 



of the foyer i'^ and it can only be augmented 

 either by increasing this dimension or b}- chan- 

 ging the focal distance of the lens, thus losing 

 a part of the light, since the divergence is 

 produced not only in the horizontal plane, — 

 the only one in which it is utilized for prolong- 

 ing the flashes, — but in every direction. With 

 the combination of vertical lenses and a c^'lin- 

 drical drum whicli serves to produce flashes 

 when electricity is used, the divergence of the 

 beams can, 'bj giving the vertical lenses a 

 proper curvature, be augmented as much as 

 desired in the horizontal plane, and the dura- 

 tion of the eclipses be diminished in propor- 

 tion, T^'hile the range of the smallest electric 

 light used will nevertheless remain much great- 

 er than that of the most powerful oil-light. 



For example : the luminous intensity' of an 

 annular panel of 45° of a first-order revolving- 

 light with a six-wick lamp equals 9,847 carcels. 

 This is the greatest intensity obtained with an 

 oil-lamp. The divergence of the beam given 

 by this same panel is 7° 7', and the duration of 

 the flash is about one-sixth part of the eclipse 

 which precedes and follows it. 



By applying the methods of M. Allard to 

 the photometric measurements of electric lights, 

 it is found that the luminous intensity' of a 

 fourth-order electric light, with a lens half a 

 metre in diameter, and fed hy a small model 

 Gramme machine, equals at least 20,000 

 carcels ; and when concentrated by means of 

 straight movable lenses in beams having a 

 divergence such that the durations of the 

 eclipses and flashes shall be the same, its in- 

 tensitj' will be equal to 40,000 carcels : that is 

 to say, that it will be four times more intense 

 than that of the most powerfnl oil-lamp, and 

 with a much shorter duration of eclipse. 



By means of electricity such immense quan- 

 tities of light are produced, that it is not 

 necessary to take into account more or less 

 beams in order to augment the range ; the only 

 object of the movable lenses being to produce 

 characteristic appearances which distinguish 

 clearh' each lighthouse from its neighbor. 

 These charactei'istic appearances, the method 

 for producing them, and the system now adopt- 

 ed in France, will be mentioned farther on. 



The different lights which serve for the light- 

 ing of French coasts are designed so as to 

 answer the different needs of navigation ; and 

 their importance varies in consequence accord- 

 ing to the role they are called upon to play, 



I The Frcncli word foyer means literally a hearlh, a place 

 tchn'r ftoviftliing Uhnriit, and, in Uic sense used liero, the source 

 of light and luat, — tlic Nparc occupied by the flame of a lamp or 

 by the electric arc. 'I'lie word is so useful that I tate the libony 

 of using it in plaee of an English paraphrase. 



of which the most important is that of sig- 

 nalling to navigators iheir approach to land : 

 and the lights constructed for this end are 

 placed in preference upon more or less ad- 

 vanced headlands ; which form, according to 

 the expression of M. L. Reynaud, " the angles 

 of a pol3"gon circumscribing all clangers." 

 These are the lights which should have the 

 greatest luminous power, and which, there- 

 fore, constitute ^rs<-orcZe)- lights. 



Between these extreme points indicating the 

 general contour of the coast, the latter still 

 presents advanced points which should become 

 centres of lights of less importance, and serve 

 to guide the vessels to their harbors. The 

 secondary lights placed on these points are 

 called second-order lights; and merit their 

 name, not only bj' their position, but also on 

 account of the less power given to their optical 

 apparatus. Along the route thus traced for 

 navigation are also found localities which it is 

 important should be pointed out to sailors : 

 these are, for example, sand-banks, sunken 

 rocks, islets, etc. From these arises the ne- 

 cessity of luminous foyers of various intensi- 

 ties, and the creation of third-, fourth-, and 

 fifih-order, and of even less powerful, lights, 

 such as are placed in harbors on the end of 

 jetties, to show vessels the entrance to the 

 channel. 



In addition, among all the lights of different 

 orders, some, placed on an island, are designed 

 to throw their light entirely around them ; 

 others, built on an advanced promontoiy or 

 established on a straight part of the coast, 

 onlj- send their rays on a fraction, more or 

 less great, of the zone which surrounds them ; 

 finally, others only have to light a determined 

 point : hence the distinction of lights in lights 

 of all the horizon, of three-fourths the horizon, 

 of two-thirds the horizon, etc. 



Until 1863 all the lights of the French coasts 

 were furnished with apparatus for oil ; and it 

 was not until this epoch that there was in- 

 stalled, at one of the two lights of la Il^ve, 

 the first apparatus for lighting by electricity. 

 After a year and a half of experiment, the 

 result having been most satisfactory, it was 

 decided to light in the same way the second 

 light of la H^ve ; and, about two years later, 

 the electric liglit was also placed in the light- 

 house at Cape Gris-Nez. Matters remained 

 in this condition until within the last few 

 years; and, while England counted on her 

 coasts six electric lights, the three which we 

 have just mentioned were the only ones in 

 existence in France. Lately, the reconstruc- 

 tion of the light-house of Planier having been 



