Qomets. 199 



m'ost, half an hour's attention. The labour is nothing, as a 

 child can turn the machine. If made longer, and divided 

 into compartments, it would serve for the preparation of 

 several kinds of paste at the same time. This machine 

 oflfers the double advantage of raising paste expeditiously 

 and to the exact degree required. — i^ei:;. Ency. Feb. 1823. 



39. Light-Houses. — One of the most recent applications 

 of the study of the properties of light, is that which is now 

 made in France in the establishments of dioptric light- 

 houses. We give this name to those in which the light of the 

 flame is not reflected, but transmitted through lenses which 

 render the rays parallel. The flame is placed in the centre 

 of eight similar lenses, and the whole system turns on its 

 axis, so that all points of the horizon are successively en- 

 lightened. The light appears alternately strong and 

 feeble ; and this intermission of splendour, weakness, and 

 disappearance, distinghishes and diversifies these lights. 

 M. Fresnel has succeeded in forming lenses of large dimen- 

 sions, by composing them of several pieces, and suppressing 

 all the thicknesses which would contribute only to the waste 

 of light, a remarkable disposition which Buffon was the first 

 to employ. 



It was especially necessary to place in the centre of illu- 

 mination a very strong light. Messrs. Arago and Fresnel 

 invented for that purpose a lamp of concentric flames, the 

 light of which is probably equal to 150 wax candles. The 

 latest experiments have proved that these lights, even in un- 

 favourable weather, are easily perceived at a distance of more 

 than eight leagues. Such is their splendour, that they were 

 even employed as signals in a geodesic operation, in which 

 Messrs. Arago and Matthieu and Messrs. Kater and Colby, 

 of the Royal Society of London, were engaged. These sig- 

 nals were seen with a telescope, more than 16 leagues dis- 

 tant, an hour before sunset ; and an hour after sundown 

 they were easily distinguished by the naked eye at that dis- 

 tance. — Fourier Rapport sur les progres des sciences mathe- 

 matiques. 



40. Comets. — The comet seen in 1822 is evidently that 

 of 1785, 1795, 1805, 1819. The duration of its revolution 

 round the sun is 1202 days. It was seen at the New Ob- 



