1814.] Limits of perpetual Snow in the North. 211 



is the estimation itself so precise as to convey an accurate idea of 

 the snow-line. It is not possible for tliis line to vary so much 

 upon the north and south sides of the same mountain. In narrow 

 and precipitous tracts, the snow lies deeper than in other places. 

 On sucli places the atmosphere is kept colder, and the approach of 

 warmer air is resisted : this is the reason why snow lies in the 

 schnt'ennilen, {suow-hvles) of the Riesengebirge, at a height of 

 only 3700 feet above the level of tlie sea : though the line of per- 

 petual congelation be 2000 feet higher than the most elevated 

 summit of these mountains. For the same reason, masses of un- 

 thawed ice occur in the mountains of Jur, at the height of only 

 3,400 feet above the level of the sea. 



The line of perpetual congelation is a crooked plane, which we 

 conceive in the atmosphere, and the snow, situated at a greater 

 elevation than this lino, never melts. But it is by no means con- 

 fined to the declivities of mountains. We seek for it on these 

 declivities, because we do not know its real elevation in the atmos- 

 phere ; and because it would be difficult for us to calculate it. The 

 sun and the shade have a much smaller effect upon this line than 

 upon the medium temperature of places. It is the business of 

 naturalists to determine the form of mountains, the nature of the 

 ground, and other causes which may make the snow in one place 

 lie deeper than usual ; while in other places, summits far above 

 the usual limit of perpetual congelation are laid bare in summer; 

 and by an examination of facts, to separate the general causes 

 from those that are merely local. When this is done, we shall 

 obtain a height for the line of perpetual congelation in the Nor- 

 wegian latitude, not varying from itself several thousand feet, ac- 

 cordii\g as we take the north and the south sides of the mountains. 



As Air. Esmaik has not fully solved the problem respecting the 

 northern snow-line, I shall take the liberty to lay before the Aca- 

 demy,* what I have been able to collect respecting it. For an 

 answer to this question, were it complete, promises a higher result 

 than the mere solution of an important physical problem. If the 

 curve or the snow-line over the earth's surface could be constructed 

 from accurate rules, its height would probably indicate the tem- 

 perature of all the places over which it passes, and thus make us 

 acquainted with the law of the change of temperature, hitherto so 

 little known. 



It appears, at first sight indeed, that this law may be determined 

 with great facility, by ascertaining the mean temperature of places 

 by means of the thermometer. But that the application of the 

 data furnished us by the thennunii'ter, is attended with very great 

 diihculty, is obvious from this striking but unfortunately true ob- 

 servation, that there are not above three, or at most four places ou 

 the earth's surface, with the mean temperature of whicii we are 

 aecuiately acquainted. 



• Thi» dikiiurlatiou wai read hcfDrc the Borlii) Aratlrmy, in iy09. 



