Comets. 199 
most, 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. his machine 
offers the double advantage of raising paste expeditiously 
and to the exact degree required. — Kev. Ency. Feb. 
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. he 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 
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 empl 
It was especially necessary to place in the centre of illu- 
mination a very strong ligh essrs. Arago and Fresnel 
_ invented for that purpose a lamp of concentric flames, the 
z of which is probably equal to 150 wax candles. The 
latest experiments have proved that these lights, even in un- 
able weather, are easily perceived at a distance of more 
Le ues. Such is their splendour, that they were 
nployed as signals in a geodesic operation, in which 
Arago and Matthieu and Messrs. Kater and Colby, 
be ae Society of London, were engaged. These sig- 
s 
a matiques. 
40. Comets.—The comet seen in 1822 ts evidently that 
ef 1785, 1795, 1805, 1819. The duration of its revolution 
round the sun is 1202 days. It was seen at the New Qb- 
. 
Mi tale 
