IN RELATION TO THE EARTH. 151 



by the ever-renewed contact of warmer air. With 

 this is connected the well-known fact, that plants 

 growing on high ground, where the air is generally 

 more disturbed than in the valleys, commonly 

 suffer less from frost. 



The low degree of heat of the higher strata of 

 the atmosphere exerts, in consequence of the great 

 mobility of the air, a very marked influence on 

 mountainous districts, whose mean temperature is 

 thus depressed below that of the plains in the 

 same latitude. This is most strikingly shown on 

 mountains seated on small horizontal areas and 

 with single peaks; their mean temperature is 

 almost entirely governed by that of the atmo- 

 sphere at the same height ; but in highland dis- 

 tricts of great extent, and on table-lands, this 

 effect of the height is also felt. 



Thus we find the mean temperature of the year 

 to be : — 



