in diminishing' the Quantity of Water in Rivers. 101 



pass through a country which has been cleared, and one which 

 has not, some time after the rainy season is over. It will then be 

 seen that the portion of his journey in the forest is still cover- 

 ed with mud, while those in the open country are completely 

 dried. It is especially in South America that the obstacles to 

 evaporation, in a region shaded with thick forests, are con- 

 spicuous. In these situations the humidity is perpetual, even 

 long after the rainy season is passed. The paths which are 

 formed through them are, during the whole year, nothing better 

 than sloughs ; and the only method of drying these forest 

 roads, is to make them as broad as 200 or 300 feet, which, in 

 fact, is a mode of clearing them. 



When it is once admitted that the running streams are di- 

 minished, as the result of clearing, it may then be important 

 to examine whether this diminution arises from the quantity of 

 rain being lessened, or from the greater evaporation, or finally 

 if it be owing to irrigation. 



I have already admitted, at the commencement of this paper, 

 that it was nearly impossible to assign any exact proportions to 

 these different co-operating causes. I shall, nevertheless, in 

 conclusion, endeavour to appreciate their respective influence in 

 a general way. And the discussion will subserve one import- 

 ant object, if I prove that there may be a diminution of the 

 running streams from the clearing alone, without the simulta- 

 neous action of the other causes. 



First, With regard to irrigation we may remark, that it is 

 necessary to distinguish between the case where extensive cul- 

 tivation takes the place of a forest, and that in which a sterile 

 district which was never wooded becomes cultivated under the 

 efforts of human industry. In the former case it is probable 

 that the irrigation will contribute little or nothing in effecting 

 any alteration in the mass of running water ; for it must be 

 generally admitted that the quantity of water consumed by the 

 vegetation of any given surface of forest, must equal, if it does 

 not exceed, that which would be absorbed by an equal surface 

 devoted to culture after it has been cleared. From this it fol- 

 lows, that the influence exerted by this cultivated district cor- 

 responds to the condition of lands which have been cleared, 

 acting solely by favouring far the evaporation of the rain- 



