DISTRIBUTION OF RAINFALL. 259 



distribution of forests in India is advanced in regard to the distribu- 

 tion of forests in America, by Mr Charles lIj,olareu, in an article on 

 America in the " Encyclopaedia Britannica." 



In this the writer says, — " We are induced to think that in all 

 countries having a summer heat exceeding 70°, the presence or 

 absence of natural woods, and their greater or less lusuriance, 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 produce 

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

 days, and withers when the moisture is exhausted; but transitory 

 rains, however heavy, will not nourish trees, because after the sur- 

 face is saturated with water the rest runs off, and the moisture 

 lodged in the soil neither sinks deep enough nor is in suffijieut 

 quantity to furnish the giants of the forest with the necessary 

 sustenance. It may be assumed that twenty inches of rain falling 

 moderately, or at intervals, will leave a greater permanent supply in 

 the soil than forty inches falling, as it sometimes dues in the torrid 

 zone, in as majiy hours. It is only necessary to qualify this con- 

 clusion by stating that something depends on the subsoil. If that is 

 gravel or a rock full of fissures, the water imbedded will soon drain 

 off; if it is clay or a compact rock, the water will rcinaiu iu the soil. 

 It must be remembered also that both heat and moisture dimiuish as 

 we ascend in the atmosphere, while evaporation iucreases ; a^d hence 

 that trees will not grow on very high ground, though its position in 

 reference to the sea and the prevailing winds siiould be favourable in 

 other respects." 



Assuming as unquestionable that the trade winds are the agents 

 which transport the moisture exhaled from its surface to the interior 

 of great continents, where it is precipitated as rain, or dew, or snow ; 

 and that mountains, by obstructing aerial curreuts and preseutiug 

 great inequalities of temperature, cause precipitation, this writei' says, 

 — " Let us consider then what will be the effect of a mural ridge like 

 the Andes in the situation which it ojuupies. In the regioa withiu 

 the 30th parallel the moisture swept up by the trade wiul from the 

 Atlantic will be precipitated, part upou the mouutains of Brazil, 

 which are but low, and so distributed as to extend far ioto the 

 interior; the portion which remains will be borne westward, and, 

 losing a little as it proceeds, will be arrested by the Andes, and fall 

 down in showers on their summits. The aerial current will now be 

 deprived of all the humidity which it can part with, and arrive in a 

 state of complete exsiccation at Peru, where no rain will consequently 



