METEOBOLOGY. 508 



pression of the surface, continues Saussure, is not merely the effect 

 of any deepening of the bed of the Rhone, by which the lake is dis- 

 charged ; it has also been produced by a diminution in the quantity 

 of water which flows into it. 



The conclusions which it seems legitimate to draw from the ob- 

 servations of Saussure are, that in the course of from 1200 to 1300 

 years the quantity of running water has sensibly diminished in the 

 districts around the Lake of Geneva. No one will, I apprehend, 

 deny that in this long period there have not been extensive clear- 

 ings of forest lands in Switzerland, and a continual increase in the 

 extent of cultivated land in this beautiful country. Here, conse- 

 quently, as elsewhere, an attentive examination of "the levels of the 

 lakes leads us to conclude, that where extensive clearings froja for- 

 est have been effected, where agriculture has extended, that there 

 has in all probability been diminution of the running waters which 

 irrigate the surface ; while in those districts where no change has 

 been effected, the amount of running stream does not appear to have 

 undergone any variation. 



The effect of forests considered in this point of view would there- 

 fore be to keep up the amount of the waters which are destined for 

 mills and canals ; and next to prevent the rain-water from collecting 

 and flowing away with too great rapidity. That a soil covered with 

 trees is further less favorable to evaporation than ground that has 

 been cleared, is a truth that all will probably admit without discus- 

 sion. To be aware that it is so, it is enough to have travelled, a 

 short time after the rainy season, upon a road which traverses in 

 succession a country that is free from forests, and one that is thickly 

 wooded. Those parts of the road that pass through the unencum- 

 bered country are found hard and dry, while those that traverse the 

 wooded districts are wet, muddy, and often scarcely passable. In 

 South America, more perhaps than anywhere else, does the obsta- 

 cle to evaporation from a soil thickly shaded with forests, strike the 

 traveller. In the forests the humidity is constant, it exists long after 

 the rainy season has passed ; and the roads that are opened through 

 them remain through the whole year deeply covered with mire : the 

 only means known of keeping forest ways dry, is to give them a 

 width of from 260 to 330 feet, that is to say, to clear the country in 

 their course. 



If once the fact is admitted that running streams are diminished 

 in size by the effect of felling the forests and the extension of agri- 

 culture, it imports us to examine whether this diminution proceeds 

 from a less quantity of rain, or from a greater amount of evapora- 

 tion, or whether perchance it maybe owing to the practice of irrigation. 



I set out with the principle that it must be next to impossible to 

 specify the precise share which each of these diflferent causes has 

 in the general result; I shall, nevertheless, endeavor to appreciate 

 them in a summary way. The discussion will have gained some- 

 thing if it be proved that there may be diminution of running streams 

 'n consequence of clearing off the forests alone, without the whole 

 of the causes being presumed to act simultaneously. 



