46 PHYSIOGRAPHIC GEOLOGY. 



tucl.es, that of extreme heat ; and the sinking of all lands would 

 diminish greatly both extremes. But sinking high-latitude lands 

 also diminishes the extreme of heat, since the lands become very 

 much heated in summer, and this heat is diffused by the winds. 

 Fuegia, on this principle, has a subalpine climate with alpine vege- 

 tation ; and Britain might approximate to the same condition if 

 the Gulf Stream could be diverted into another ocean. 



The mean temperature of the Northern hemisphere is stated by 

 Dove at 60° F., and of the Southern at 56° F., while the extremes 

 for the globe, taking the annual means, are 80° F. and zero. If there 

 were no land, the mean temperature would probably be but little 

 above what it is now, or not far from 60° for the whole globe. 



6. DISTRIBUTION OF FOREST-REGIONS, PRAIRIES, AND 

 DESERTS. 



48. The laws of the winds are the basis of the distribution of 

 sterility and fertility. 



1. The warm tropical winds, or trades, are moist winds ; and, 

 blowing against cooler land, or meeting cooler currents of air, they 

 drop the moisture in rain or snow. Consequently, the side of the 

 continents or of an island struck by them — that is, the eastern — 

 is the moister side. 



2. The cool extratropical winds from the westward and high 

 latitudes are only moderately moist (for the capacity for moisture 

 depends on the temperature) ; blowing against a coast, and bending 

 towards the equator, they become warmer, and continue to take up 

 more moisture as they heat up ; and hence they are drying winds. 

 Consequently, the side of a continent struck by these westerly cur- 

 rents — that is, the western — is the drier side. 



There is, therefore, double reason for the difference in moisture 

 between the opposite sides of a continent. 



Consequently, the annual amount of rain falling in tropical 

 South America is 116 inches, while on the opposite side of the 

 Atlantic it is 76 inches. In the temperate zone of the United 

 States east of the Mississippi, the average fall is about 44 inches ; 

 in Europe, only 32. America is hence, as styled by Professor 

 Guyot, the Forest Continent ; and where the moisture is not quite 

 sufficient for forests, she has her great prairies or pampas. 



The particular latitudes of western coasts most affected by the 

 drying westerly winds — those between 28° and 32° — are generally 

 excessively arid, and sometimes true deserts. (W. C. Eedfield, in 

 Amer. Jour. Sci. xxv. 139, 1834, and xxxiii. 261, 1838.) 



