1849.] 



exercised hy Trees on Climale. 



435 



region shaded with thick forests are conspicuous. In these situa- 

 tions the humidity is perpetual, even long after the rainy season is 

 past. 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 method of clearing them. 



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

 ed as the result of clearing, it may then be important to examine 

 whether this diminution arises from the quantity of rain being les- 

 sened, or from the greater evaporation, or finally if it be owing to irri- 

 gation, 



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 important object if I prove that 

 there may be a diminution of the i;jinning streams from the clearing 

 <alone without the simultaneous action of the other causes. 



First, with regard to irrigation we may remark that it is necessa- 

 ry to distinguish between the case where extensive cultivation takes 

 the place of a forest and that in which a sterile district which was 

 never wooded becomes cultivated under the efforts of human indus- 

 try. In the former case it is probable that the irrigation will contri- 

 bute little or nothing in effecting any alteration in the mass of run- 

 ning 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 follows, that the influence exerted by this cultivated district 

 corresponds to the condition of lands which have been cleared acting 

 solely by favouring the evaporation of the rain water. In the latter 

 case again that is to say where a great extent of uncultivated land 

 shall have been reclaimed, there wdll be an evident consumption of 

 water by the vegetation which has been there promoted : and the 

 introduction of agricultural industry will under these circumstances 

 tend to diminish the water-courses which traverse the country. It 

 is very probable that we are to attribute to this circumstance the 

 gradual drying up of the lakes which to a certain extent gauge the 



TOL, Xy. NO. XXXVI. ^ 1 



