236 Prof. Hansteen on the Fog' of the Polar Regions. 
the same manner has Nature provided for the cooling of the ani- 
mal body, in the time of excessive heat, by the increased per= 
spiration, the evaporation of which preserves it in an almost un- 
changed temperature. Now, as the polar lights are an ex- 
pansible substance, which, in regions surrounding the magnetic 
poles, is continually issuing from the surface of the earth, even 
at times when we who lie at a distance from these points perceive 
nothing of this, it may be conceived, that this stream, in passing 
through the atmosphere, is continually lowering its temperature, 
and thus, in decomposing the watery vapours, producing fog. 
In this manner, too, may be explained another well known pheno- 
menon. It might be expected, that, in the same parallels, the 
mean temperature would be every where nearly the same, as 
the length of the day, and the height of the sun, depend alone 
on the latitude. But this is by no means the case. In that 
parallel which passes the 60th degree of north latitude, the tem- 
perature is the lowest in Hudson’s Bay, and at the south 
poiitt of Greenland. As we approach the coasts of Norway it 
rises, and in Christiania and Stockholm, is about H- 5°, In 
Petersburgh it has begun to descend, and the farther we ad- 
vance to the east it becomes the lower ; so that a Siberian 
cold has become a proverbial expression. Although Cape 
Horne lies in the 56th degree of south latitude, that is 
about the same latitude with Copenhagen, the cold there is in- 
tolerable ; and yet at other points of the same parallel it is less. 
It seems also pretty well proved, that, in one and the same 
parallel, the mean temperature is least in the meridians which 
pass through the magnetic poles. Count Humboldt has rem 
dered this distribution of temperatures subject, in a very remark- 
able manner, to our observation, by connecting, on a chart, all 
the points where the mean temperature is the same, by a sort of 
curve lines, which he calls Isothermic Lines. These lines are 
by no means parallel to the equator, as might be supposed, but 
remove to the greatest distance from it in the Atlantic Ocean, 
sinking again both in America and in the eastern hemisphere. 
Comparing this chart with the lines of the inclination of the 
needle, in the seventh table of my Magnetismus der Erde^ it 
will be found that these different lines follow very much the 
same course ; a circumstance which is to be explained in this 
