434 



Notes on the Tnjlaence 



[No. 36, 



speedily the current of waters occupied no greater space than the 

 bottom of the valley ; in short, the lake of Geneva was then formed. 

 But turning from this to the monuments which have been construct- 

 ed by man it is impossible to doubt that during the course of twelve 

 or thirteen centuries, the waters of the Lake of Geneva have con- 

 siderably retired. This is evident from the flat shores it has left 

 near its margin, and, even in the town itself, the Qu artier de Rive 

 and the low streets have been built upon such sites. This lowering 

 of the surface, continues Saussure, is not the result only of the wear- 

 ing down of the channel whence its waters issue, it has likewise been 

 produced by diminution of the quantity of the waters which flow into 

 it. The general result which maybe safely drawn from the observa- 

 tions of Saussure is that during the period of twelve or thirteen cen- 

 turies, the running streams have gradually diminished throughout the 

 districts in the neighbourhood of the lake of Geneva. And there is 

 no one, I believe, will dispute that during that long period, there 

 has been a vast clearing away of wood and a rapidly increasing 

 advance in the cultivation of this beautiful country. Upon the 

 whole, by the examination of the levels of lakes, we have arrived at 

 this conclusion, that in those countries which have been extensively 

 cleared, it appears very probable that there has been a diminution 

 of the running streams which flow through the district, whilst, on 

 the other hand, where no great change has been effected in this way, 

 the streams have been subjected to no variation. - 



Great forests, therefore, in the point of view we are now regard- 

 ing them appear to have the effect, first, of preserving the volume of 

 water destined for the use of machines and of canals, and then to be 

 an obstacle to the rain water collecting and running off with too 

 much rapidity, being at the same time an obstacle to evaporation. 

 That a surface covered with trees will not be so favourable to evapo- 

 ration as a well wooded one is what every one will admit, without 

 discussion. But that the difference of these two conditions may be 

 adequately appreciated, it is necessary that the traveller should suc- 

 cessively 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 covered 

 with mud while those in the open country are completely dried. It 

 is especially in South America that the obstacles to evaporation in a 



