504 METEOROLOGY. 



With regard to irrigation, it is necessary to distinguish bct\^een 

 that case in which an extensive farm has been substituted for an im- 

 penetrable forest, and that in which an arid soil, which never sup- 

 ported wood, has been rendered productive by the industry of man. 

 In the first case it is very probable that irrigation will have contri- 

 buted but little to the diminution in the mass of running water ; it 

 may readily be imagined that the quantity of water used up by a 

 dense forest will equal, at all events, if not exceed, that which will 

 be required by any of the vegetables which human industry substi- 

 tutes for it. In the second case, that is to say, where a great extent 

 of waste country has been brought under cultivation, there will evi- 

 dently be consumption of water by the vegetation which has been 

 fostered upon the surface ; agricultural industry will thus tend to 

 diminish the quantity of water which irrigates a country. It is ex- 

 tremely probable that it is to a circumstance of this kind that we 

 must ascribe the diminution of the lakes which receive so large a 

 proportion of the running streams of the north of Asia. It is al- 

 most unnecessary to add, that in circumstances of this kind the effect 

 which is due to the simple evaporation of rain-water is not increased ; 

 the loss by this means must be rather less, because from a surface 

 covered with plants evaporation takes place more slowly than from 

 one that is devoid of vegetation. 



In the considerations which I have presented upon the lakes of 

 Venezuela, of New Granada, and of Switzerland, the diminution may 

 be directly ascribed to a less mean annual quantity of rain ; but it 

 may with equal reason be maintained to be a simple consequence of 

 more rapid evaporation. 



There arc, in fiict, a variety of circumstances under the influence 

 of which the diminution of running streams can be shown to be con- 

 nected with more active evaporation. 1 shall confine myself to the 

 mention of two particular instances, one noticed by M. Desbassyns 

 de Richemond, in the Island of Ascension ; the other is from obser- 

 vations by myself, and is among the number of facts which I regis- 

 tered during my residence for several years at the mines of ISIar- 

 mato. 



In the Island of Ascension there was an excellent spring situated 

 at the foot of a mountain originally covered with wood ; this spring 

 became scanty and dried up after the trees which covered the moun- 

 tain had been felled. The lot^s of the spring was rightly ascribed to 

 the cutting down of the timber. The mountain was therefore plant- 

 ed anew, and a few years afterwards the spring reappeared by de- 

 grees, and by and by flt)wed with its former al)undance. 



The metalliferous mountain of Marmato is situated in the provmco 

 of Popayan, in the midst of immense forests. The stream along 

 which the mining works are established is formed by the junction of 

 several small rivulets which take their rise in the taljlc-land of !San 

 Jorge. The country whicii overlooks the establishment is thickly 

 wooded. 



In 1826, when I visited the mines for the first time, Marmato con- 

 sisted of a few miserable cabins, inhabited by nefjro slaves. In 



