1814.] and on Vegetation. 29 



or smaller degree, the permanent temperature of the earth : but it 

 is surprising to how small a depth in the earth the great alterations 

 in the temperature of the air are able to penetrate. Indeed, they 

 are almost entirely confined to the surface. The small spring of 

 Itassbybackar appears, from an examination of the spot, scarcely to 

 reach a foot below the bottom. Larger springs may go some feet 

 deeper. However, from their temperature the influence of the 

 constant temperature of the earth is not to be learned. Several 

 trees and perennial plants shoot their roots much deeper, and there- 

 fore reach a still more uniform temperature. The roots of the 

 palms, for example, penetrate several fathoms perpendicularly 

 downwards into the soil : and this may probably be the reason why 

 the palm, when compared with so many other families of plants, 

 i- confined to so narrow a zone ; for it is only in very hot climates 

 that the roots at such a depth can meet with the temperature requi- 

 site for the vegetation of palms. This is probably, likewise, the 

 reason why it is so much easier for us to raise in our gardens annuals 

 than perennial plants. The latter, in consequence of the greater 

 depth of their roots, feel more the effects of our cold climate than 

 the former. 



The same species of trees cease to vegetate on the mountains in 

 warmer countries at a much higher temperature : so that their dis- 

 tance from the snow line is twice as great as with us; but similar 

 herbs in both places rise to the same distance from the snow line. 

 On the southern mountains the difference of the various seasons of 

 the year is very small : the heat of the summer dots not produce 

 such an effect upon the soil as it docs farther north : hence the trees 

 which, sinking deeper into the earth than the herbs, require a higher 

 temperature in the soil, cannot approach so near the snow line as 

 they can in higher latitudes. Probably this greater difference in 

 the seasons of the year is the reason why the temperature of the 

 earth in the north is higher than the mean temperatuie of the 

 atmosphere.* It would appear from the obseivntions of Von 

 Humboldt that the contrary is the case in warm climates ; for the 

 sun's heat -hows its intensity and duration upon the dry soil, while 

 the winter's cold, in consequence of the coat of snow with which 

 the earth is covered, acts but feebly upon the soil.f 



The influence of the sun's heat is generally very different upon 

 wet and dry soils. In the latter it penetrates to a considerable 

 depth; but not in the former. Moving water in hikes and rivers 

 very readily acquires the temperature of the atmosphere ; but it 

 prevents that temperature from penetrating deeper, in consequence 



• Mean temperature of the atmosphere at Upsala, from SO years' 



ouservalioul <l?-o ; • 



Temperature of the earth at Upsala 43'70 



/ "it litich. 



+ Dor'uip winter, likewise, the greatest eebicleof the equalization of i mpe- 

 rature it vvanliDe;, nainel>, r.<- veins and drnp^ ol n.iici ipreadiug over tiie \\liole 

 toil with the teoipcraluie of the surface. — fun Jlwh. 



