GENEEAL CONCLUSIONS. 195 



extent of heated ground, and whicli otherwise might be precipi- 

 tated and form rain, is dissipated and carried of. Thus the clouda 

 that gather round mountains are seen to vanish as they pass over 

 the plains below. The forest does not become heated by the sun, 

 and therefore does not radiate heat enough to dissolve the vapor 

 in the atmosphere above it, while the open ground is warmed by 

 the sun and radiates heat into the air which drifts over it. The 

 lower limit of perpetual snow is said to be from two thousand to 

 four thousand feet higher on the northern than on the southern 

 slope of the Himalayas. The explanation of this apparently 

 anomalous fact is found partly in the condensation and con- 

 gelation of the moisture with which the southern monsoon is 

 charged, and its precipitation as snow on the southern scarp of 

 this mighty chain. But the configuration of the earth's surface 

 in this region furnishes another explanation to which physical 

 geographers formerly ascribed even greater importance. This 

 lofty chain is skirted on the north by the extensive, high plateau 

 of Thibet which is without forests, and it was believed that 

 the naked surface of this plateau emitted, by reflection and 

 radiation, heat enough to dissolve the moisture of the atmospheric 

 strata above it, and thus to prevent the formation of rains and 

 snows. Our vast "Western fields and plains, though lower by 

 10,000 feet, exercise a similar function, and probably materially 

 modify the amount of precipitation in our chmate. Whether the 

 KussisiTi' steppes exert a like influence is a question which I have 

 not seen discussed. 



Total OUmatio Injhjbence of the Forest. 



Aside from the question of local disturbances and their com- 

 pensations, it does not seem probable that the forests sensibly 

 affect the general mean of atmospheric temperature of the globe, 

 or the total quantity of precipitation, or even that they had this 

 influence when their extent was vastly greater than at present. 

 The waters cover about three-fourths of the face of the earth, and 

 if we deduct the frozen zones, the peaks and crests of lofty 

 mountains and their craggy slopes, the Sahara and other great 

 African and Asiatic deserts, and all such other portions of the 

 solid surface as are permanently unfit for the growth of wood, we 



