452 SEC. 12. APPLIED MECHANICS. 



tried in public on 13th April 1821. It eras placed on the top of the Obser- 

 vatory buildings, together with two large reflectors, one by Lenoir, the other 

 by Bordier-Marcet. The Lighthouse Committee, of which Mr. Becquey, 

 director-general of the " Fonts et Chaussees," was chairman, went to the 

 summit of Montmartre to judge of the effect. The result confirmed the 

 inventor's previsions, and every one allowed the superiority of the lens over 

 the reflectors. Meanwhile, Fresnel had already thought of improving these 

 first essays. He had invented a machine for constructing circular rings, and 

 M. Soleil was instructed to make eight large lenses constructed on annular 

 principles. He soon finished some of them, and in September 1821 the 

 Lighthouse Committee determined to try their effect at long distances. 

 Fresnel had fitted up on the top of the " Arc de 1'Etoile " a revolving appa- 

 ratus upon which were fixed two of these annular lenses, four polygonal 

 lenses previously constructed, and four semi-polygonal lenses. At the focus, 

 a four-wicked lamp was burning. The committee then went to Chateney, a 

 village situated N.N.E. of Paris, 24^ kilometres distance from the " Arc de 

 1'Etoile." The experiment took place during the night of 7/8 September 

 1821, and the results were adjudged as very satisfactory. The eight annular 

 lenses that had just been constructed form part of the first flashing-light 

 apparatus of the 1st class that Fresnel himself fixed on the watch-tower of 

 Cordouan, and which has lighted the entrance to the Gironde for more than 

 30 years. (The lens exhibited under No. 3 comes from this apparatus.) 



If an idea is to be formed of the progress. made in the science of lenticular 

 lighthouses from its origin to later times, the three lenses before mentioned 

 should be compared with the lenses of modern construction exhibited under 

 Nos. 11, 12, and 13. 



Fixed Light Apparatus. 



No. 4. First fixed light apparatus of 0'50 m diameter, invented by A. Fresnel, 

 and constructed in 1824. 



Fresnel, after his appointment to the Lighthouse Committee of 1819, first 

 gave his attention to flashing lights ; meanwhile, he had thought about ob- 

 taining fixed lights, and in the first design of the lenticular lighthouse that he 

 submitted to the committee on 31st October 1820, he indicated, as a solution, 

 the use of cylindrical lenses ; but the Lighthouse Committee had thrown 

 aside the fixed light system as possessing less reverberating power that 

 revolving lights, and as being liable to be mistaken for the incidental lights of 

 the coast. The committee altered this decision later, and Fresnel then in- 

 rented the system of fixed light apparatus (0.50 m diameter), exhibited under 

 No. 3. In this apparatus the lenticular drum, which should be cylindrical, 

 so as to give an uniform subdivision of light, shows a polygonal form of 

 16 fasces, because no lathe was then known for making cylindrical pieces. 

 The upper part is made up of two lenticular zones in the shape of a 16-panel 

 cupola, every element of which is coupled with a plane mirror. The lenses 

 unite in parallel fasces the rays emitted by the light, and the mirrors reflect 

 them in the direction of the horizon. 



A similar system, but having one lenticular zone only, is fitted at the lower 

 part. The lamp has two wicks, and stands upon a plate raised or lowered by 

 a jack between three leaders. With the polygonal form that had to be 

 adopted, there were 16 directly receiving more light than the intermediate 

 parts, but Fresnel, while constructing the instrument, discovered the means of 

 greatly lessening this inequality, by alternating the shining directions of the 

 lenticular cylinders with those of the two ether parts. This first trial of a 

 fixed light apparatus was demonstrated by Fresnel before the Academy of 

 Sciences of Paris, at their sittings of 3rd May 1824. It was then inaugurated 

 on the 1st February 1825 in the port of Dunkirk. 



