16 GEOLOGY OF SOIL. 



Mean temperature, Fahr. of tho 

 Latitude. Year. Summer. Winter. 



Scotland, (Inverness) 58° +46*3 +57-3 +36*5 



Norway, (Drontheim) 64° +39*5 +59 -f 23 5 



Sweden, 62° +39-5 +59 +23*5 



St. Petersburgh, 60*25+38 +60-8 +15 6 



North latitude 64°, appears then, to be the utmost 

 limit of wheat. It is evident by inspection, that 

 thi-> is not determined by the cold of winter ; for 

 spring wheat would not be affected by it; and even 

 if sown in autumn, in these far northern regions, the 

 seed would be effectually preserved from the rigors 

 of winter, by that thick mantle of snow, which be- 

 comes thicker and more lasting towards the north. 

 The temperature of the air exerts no influence on 

 seeds of plants buried under snow. Nor does the 

 mean temperature of the year exert any effect ; it 

 is seen ranging 9°, w ; hile the summer temperature 

 varies only 3 1-2°. The summer temperature alone 

 defines the limit of northern wheat cultivation, and 

 this is an isotheral' line of 57-4°. Yet it is found, 

 that there are places, where, as in Russia, the means 

 of spring and autumn, both depending on that of 

 winter in part, are too low to allow wheat to be rais- 

 ed under this line of 57-4 degrees. In truth, the 

 relation of climate to cultivation cannot be accurate- 



