794 On the Influence of Forests on Climate. [Aug. 



sence or absence of natural woods, and their greater or less luxuriance, 

 may be taken as a measure of the amount of humidity, and of the 

 fertility of the soil ; short and heavy rains in a warm country will pro- 

 duce grass, which having its roots near the surface springs up in a few 

 davs, and withers when the moisture is exhausted, but transitory rains, 

 however heavy, will not nourish trees, because, after the surface is 

 saturated with water, the rest runs off, and the moisture lodged in the 

 soil neither sinks deep enough, nor is in sufficient quantity to furnish 

 the giants of the forest with the necessary sustenance. It may be 

 assumed that 20 inches of rain falling moderately, or at intervals, will 

 leave a greater permanent supply in the soil than 40 inches falling as it 

 sometimes does in the torrid zone in as many hours." 



" In all regions," he continues, " where ranges of mountains inter- 

 cept the course of the constant or the predominant winds, the country 

 on the windward side of the mountains will be moist, and that on the 

 leeward dry, and hence parched deserts will generally be found on the 

 west side of countries within the tropics, and on the east side of those 

 beyond them ; the prevailing winds in these cases being generally in 

 opposite directions. On this principle the position of forests in North 

 and South America may be explained. Thus for example, in the region 

 within the thirtieth parallel, the moisture swept up by the trade wind 

 from the Atlantic is precipitated in part upon the mountains of Brazil, 

 which are but low and so distributed as to extend far into the interior. 

 The portion which remains is borne westward, and losing a little as it 

 proceeds, is at length arrested by the Andes, where it falls down in 

 showers on their summits. The Aerial current, now deprived of all 

 the humidity with which it can part, arrives in a state of complete ex- 

 siccation at Peru, where consequently no rain falls. In the same 

 manner the Ghauts in Hindoostan, a chain only three or four thousand 

 feet high,* intercept the whole moisture of the atmosphere, having 

 copious rains on their windward side, while on the other the weather 

 remains clear and dry. The rains in this case change regularly from 

 the west side to the east, and vice versa with the monsoons. But in 

 the region of America, beyond the thirtieth parallel, the Andes serve 

 as a screen to intercept the moisture brought by the prevailing winds 

 from the Pacific Ocean ; rains are copious on their summits, and in 



* Five or six thousand ? 



