,544 SEC. 12. APPLIED MECHANICS. 



of a circle having, like the rings, its centre on the optical axis ; the result 

 is that the rings can remain intact, instead of having to be cut, as was the case 

 until now. 



5th. The lamp, placed at the focus of the lens exhibited, shows special 

 arrangements, due to M. Denechaux. 



Thus, the skin pockets or valvulae, and the leathern valves, which are 

 sometimes the cause of disorder, are replaced by ordinary pistons aod metal 

 valves. This system has produced good results in experiments made at the 

 depot, but it has not yet received practical sanction. 



The lamp with five wicks for burning mineral oil has an intensity of 

 36 carcel burners, the fixed light apparatus produces an intensity of 640 

 burners, and the annular lenses of more than 5,000 burners. 



No. 13. Apparatus for electric revolving light, constructed by Messrs. 

 Santter, Lemonnier, & Co., 1876. 



This instrument is intended to produce, by electric light, a light revolving at 

 intervals of 30 seconds. It includes a fixed light apparatus 50 m diameter, 

 lighting three-fourths of the horizon, around which revolves, in eight minutes, 

 a tambour of 62 m diameter, and composed of 16 vertical, lenticular elements. 



In the section of the fixed light apparatus the central dioptric part sub- 

 tends a vertical angle of 76 degrees, which is greater than in the old sections. 

 This arrangement is adopted in order that the luminous ray may met-t the 

 last dioptric element at the same angle as the first catadioptric ring, and should 

 surfer no more loss by reflection upon the one than upon the other. The 

 apparatus having to be fixed on an elevated point, the section of the several 

 parts, except that of the two lowest catadioptric rings, has been calculated so 

 as to throw the focal line of the emergent rays, 30 minutes below the horizontal 

 line ; in the calculation of the t\vo lowest rings, this angle is increased by 

 three degrees for the last but one, and by five degrees for the last, so that the 

 lighthouse may remain visible at a short distance, that is, by a navigator 

 placed below the divergent cone emitted by the rest of the apparatus. 



The sixteen vertical lenses are contiguous, and are each compose! of a 

 single element, about 12 m wide, the curve of which has been calculated so as 

 to give with the electric light an horizontal divergence of three degrees seven 

 minutes. The duration of a flash is, accordingly, of about five seconds, and 

 the interval between the end of a flash and the beginning of the following- 

 one is 25 seconds. 



The maximum intensity of the flash rises to about 60,000 carcel burners, 

 assuming at the focus an electric light of 200 burners power. 



The electric light is produced in this apparatus, as in the lighthouses 

 established on the coasts of France, by means of a Serrin regulator and an 

 electrical machine of the Compagnie PAlliance. 



Experiments have been made with the Serrin regulator at the lighthouse 

 depot since the year 1860. A model on a large scale has been constructed 

 especially for the lighthouse service, and has always given good results. The 

 regulator exhibited is a counterpart of this model. 



The electro-magnetic machine has been, as is well known, designed by MINT. 

 Nollet and Joseph Van Malderen, in accordance with the same principle as 

 the scientific apparatus of Pixii and Clarke. It produces alternate currents, 

 and, as it was in the first instance destined for the decomposition of water cr 

 for electro metallurgy, it was provided with a commutator for bringing the 

 currents into one constant direction. When the question was raised of apply- 

 ing it to the production of light, M. Van Malderen, who had then become the. 

 mechanical engineer to the Compagnie PAlliance, conceived the happy ide"a of 

 suppressing the commutator, which is difficult to maintain, and has the effect 

 of more or less weakening the current. The luminous intensity was found to 

 be appreciably augmented, and the fact was soon acknowledged that alternate 



