OF THE AURORA BOREALIS ON THE MAGNETIC NEEDLE. 
107 
observations of the 20th December throw a satisfactory light on this subject, 
and shall now briefly state the conditions of the phenomena of that evening, 
and their localities, which lead me to this conclusion. 
The mass of hills, named Coreen, to which both the clouds and the Aurora of 
that evening were entirely confined, is eight or ten miles long from E. to W., 
and about four miles broad in some places from N. to S. It is nearly bisected 
by the magnetic meridian of this place, which cuts off the broadest and highest 
part of it to the W. About twelve or fifteen square miles rise above the 
limits of cultivation ; that is, about six or seven hundred feet above the bot- 
toms of the conterminous valleys ; and some of the summits attain an elevation 
of nine hundred and one thousand feet. 
On the W., a little by S., of this mass of hills, there extends for eighteen or 
twenty miles a succession of valleys of considerable width, all bounded on 
the S. and N. by high lands of great extent, and separated from each other by 
comparatively low eminences. Through these the river Don flows from W. to E. 
On descending this succession of valleys with the course of the river, the Coreen 
hills are seen directly in the line of them, and apparently shutting them nearly 
up at the E., the extension of the low ground turning there, for some distance, 
suddenly N. and afterwards E., by the N. side of these hills ; and the river, on 
the contrary, when it reaches these hills, turning several points S. of its former 
course, and entering this valley of Alford at its N. W. corner, by a very narrow 
defile between Coreen and another hill on the S., of nearly equal elevation but 
comparatively little extent, which is placed more in the direction of the 
general outline of the high lands bounding the valleys on the south. 
We have then, in the situation of the Coreen hills, a most satisfactory ex- 
planation of the formation of clouds over them on the 20th December, in well 
established principles independently of the Aurora. The wind coming steadily 
the whole evening from W. carried the air out of the upper Don valleys, 
(where, at a comparatively low elevation, it was nearly saturated with aqueous 
vapour, to which the river in its long course would much contribute,) over the 
surface of Coreen ; that is, lifted the whole mass several hundred feet perpen- 
dicularly, and the diminished pressure and consequent expansion lowered its 
temperature to the point of saturation. 
As the Aurora was entirely confined to the region of the cloud, the cause of 
p 2 
