Atmosphere and the Earth. ^^^ 



they would be if this Hne coincided with the terrestrial equator, 

 and deviate little from 81-5; that is_, the mean temperature of 

 the air in these regions. 



The temperature of the earth stands in manifest connexion 

 with other appearances exhibited by nature. I v/ill here only 

 mention some of these, to show how fruitful these considera- 

 tions may in time become. Wahlenberg has already shewn, that 

 in high latitudes many perennial deeprooted herbs, trees and 

 shrubs, only thrive, because the temperature of the earth exceeds 

 the mean temperature of the air. In these latitudes, the periods 

 of vegetation appear to be as much regulated by the periods in the 

 temperature of the earth as that of the mean temperature of the 

 air ; a remark which I had often an opportunity of making, on 

 my journey to the northern Urals. In Middle Russia, the ve- 

 getation commences later than in Germany, but the harvest falls 

 at the same time, in the month of July ; but if, proceeding north- 

 wards, we pass the point where the mean temperature of the air 

 is 32-0, the harvests become later, and happen in August ; and, 

 finally, before the cultivation of grain is completely at an end, 

 in the beginning of September; this period, which corresponds 

 with the maximum of the atmospherical temperature, approaches, 

 therefore, in high latitudes, the time when the terrestrial tempera- 

 ture is highest. The connexion which the direction of the north- 

 ernmost isogeothermal lines appears to maintain with the boun- 

 dary of the polar ices, also merits our attention. These limits 

 are laid down on the accompanying chart, from Scoresby's in- 

 teresting paper on the Polar Ice *. The first glance at the chart 

 informs us, that the isogeothermal line of 32° extends somewhat 

 to the south of the boundary of the ice, except at Greenland : but 

 of this country we know that formerly it was not so much beset 

 with ice as at present. The terrestrial temperature can only 

 operate on masses of ice, which sink to a considerable depth, 

 which is not the case with those on the Continent ; and iience the 

 effect of a large mass of land, such as Greenland, on the polar ice 

 boundary is easily explained. The floating of die ice on the east 

 coast of Greenland towards the south-west, which Scoresby has so 

 well observed, would intimate colder points in the north of Ame- 

 rica, particularly of Grt>enland ; at least, I do not know how we 



" This remarkable memoir first appeared in the 2il volume of the Memoirs 

 of the Wemerian Society. 



