44 PHYSIOGRAPHIC GEOLOGY. 



Great Britain is tempered in its climate by its winds and the oceanic current (the 

 Gulf Stream). Fuegia, which is almost surrounded by water, also has an insular cli- 

 mate, — the winter's cold falling little below 32°, although below 53° S. latitude. 



3. Absence of land from high latitudes is equivalent to an absence 

 of the source of extreme cold ; and from tropical latitudes, that of ex- 

 treme 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 sub al- 

 pine climate with alpine vegetation ; and Britain might approximate 

 to the same condition if the Gulf Stream could be diverted into an- 

 other ocean. 



The mean temperature of the Northern hemisphere is stated by 

 Dove at 60° F., and of the Southern at 5&° F., while the extremes for 

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

 Vvere 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. 



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, blow- 

 ing against cooler land, or meeting cooler currents of air, they drop 

 the moisture in rain or snow. Consequently, the side of the conti- 

 nents 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 lati- 

 tudes are only moderately moist (for the capacity for moisture depends 

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

 equator, they become warmer, and continue to take more moisture as 

 they heat up ; and hence they are drying winds. Consequently, the 

 side of a continent struck by these westerly currents — that is, the 

 western — is the drier side. 



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

 tween the opposite sides of a continent. 



Consequently, the annual amount of rain falling in tropical South 

 America is 1 1 6 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 Conti- 

 nent ; and, where the moisture is not quite sufficient for forests, she 

 has her great prairies or pampas. 



