THE GREAT FOREST-BELTS. 29 



belt and giving up its store of vapour, flows off north 

 and south as dry, cool air, and descends to the earth in 

 the vicinity of the tropics. Here it sucks up whatever 

 moisture it meets with and thus tends to keep this zone 

 in an arid condition. The trades themselves are believed 

 to be supplied by descending currents from the tem- 

 perate zones, and these are at first equally dry and 

 only become vapour-laden when they have passed over 

 some extent of moist surface. At the solstices the sun 

 passes vertically over the vicinity of the tropics for 

 several weeks, and this further aggravates the aridity ; 

 and wherever the soil is sandy and there are no lofty 

 mountain-chains to supply ample irrigation the result is 

 a more or less perfect desert. Analogous causes, which 

 a study of aerial currents will render intelligible, have 

 produced other great forest-belts in the northern and 

 southern parts of the temperate zones ; but owing to the 

 paucity of land in the southern hemisphere these are 

 best seen in North America and Northern Euro-Asia, 

 where they form the great northern forests of deciduous 

 trees and of Coniferse. These being comparatively well- 

 known to us, will form the standard by a reference to 

 which we shall endeavour to point out and render in- 

 telligible the distinctive characteristics of the equatorial 

 forest vegetation. 



General Features of the Equatorial Forests. It is not 

 easy to fix upon the most distinctive features of these 

 virgin forests, which nevertheless impress themselves upon 

 the beholder as something quite unlike those of tempe- 

 rate lands, and as possessing a grandeur and sublimity 

 altogether their own. Amid the countless modifications 

 in detail which these forests present, we shall endeavour 



