flected image of the sun; and that there is no observation of this 
TRANSACTIONS OF THE SECTIONS. 31 
___ It may be superfluous to point out, to persons disposed carefully to 
_ observe this phenomenon, the necessity of watching for every indica- 
tion of its appearance—the low dark arch in the north, for example, 
- one of the most infallible—or the importance of noting, as accurately 
as possible, the time, bearing, and altitude of each particular appear- 
ance; but I would especially call their attention to every circumstance 
_eonnected with the dark arch ; the first indications of its formation ; 
the manner in which it occasionally breaks up; whether irregularities 
in its form are always attended with coruscations; whether, as it has 
appeared to me, the matter of which it is composed does commonly 
- rush through the luminous bands; whether, in short, all the pheeno- 
- mena will warrant the conclusion, that the matter which, during an 
_ Aurora, appears in the form of a dark low arch, is different from that 
_ forming the luminous bands, and that the different phenomena are 
due to the action of the same cause, on two aeriform masses, which 
have distinctive characters with reference to such action. 
METEOROLOGY, &c. 
On M. Poisson's Theory of the Constitution of the Atmosphere. By 
J. W. Luszock, F.R.S., ge. 
He commenced by observing, that at a late meeting of the Section,* . 
M. De la Rive described a very curious phenomenon presented by 
Mont Blanc after sunset, which consisted in the re-appearance of the red 
 ¢olour of the snow, produced by the rays of the setting sun. Mr. Lub- 
_bock said that he should now venture, with great diffidence, to submit 
the possibility of the following explanation. 
M. Poisson considers it a necessary consequence of the laws of equi- 
librium of elastic fluids, that the atmosphere of the earth, at a certain 
height, becomes liquefied by cold.—(Traité de Mécanique, vol. ii. p. 
_ 612;+ Théorie Mathématique de la Chaleur, p. 460, et Supplement, 
_ note D.)—In this way the atmosphere receives an abrupt termination; 
_ without which, indeed, it would be difficult to imagine that the planets - 
and comets move in space devoid of considerable resistance. If the 
atmosphere be constituted as M. Poisson infers from analysis, it seems 
_ to me, Mr. Lubbock observed, that we might expect that the phzno- 
menon described by M. De la Rive would take place, and that the 
image of the sun, reflected from the interior surface of the liguid air, 
would be reflected again to the observer, after senset, by the moun- 
tain. On the other hand, it may be stated, that an observer sta- 
tioned, at sunrise or sunset, upon Mont Blane, or in a balloon, or in 
any position sufficiently elevated, ought to see in the sky the re- 
* See p. 10 of these Transactions. Y 
+ ‘‘ Ainsi, pour fixer les idées, on peut se représenter une colonne atmosphérique 
qui s’appuie sur la mer, par exemple, comme un fluide élastique terminé par deux 
ides, dont l’un a une densité et une température ordinaire, et l’autre une tempé- 
rature et une densité excessivement faibles.” 
