INFLUENCE OF AGRICULTURE ON CLIMATE. 497 



changed. In the following remarks, I shall attach myself particu- 

 larly to observations upon lakes which have no outlet, by reason of 

 the facility with which any even slight change in the level of these 

 must be discovered. I shall not, however, neglect those lakes 

 which have an exit by a stream or canal, because I believe that the 

 study of these may also lead to accurate enough results ; the only 

 point requiring preliminary remark, is the sense in which the words, 

 change of level, are to be taken. 



Geologists admit, that the level of the waters upon the surface of 

 the globe has everywhere undergone great changes, whether atten- 

 tion be directed to the shores of the sea or to those of great inland 

 lakes. This fact is universal, and is questioned by none, but great 

 diversity of opinion prevails in regard to the cause of the phenome- 

 non. Some pretend, that in many cases the change of level is only 

 apparent, — that the body of water has not sunk, but that the shores 

 have risen ; others, again, maintain that there has been a true dimi- 

 nution in the mass of fluid, a true drying up, and that its level has 

 actually sunk. I shall not, in this place, enter upon the great geo- 

 logical question ; the variations which are there signalized are often 

 of vast extent, and involve the supposition of violent catastrophes, 

 which, in a general way, were long anterior to the historical epoch. 

 I shall only refer to changes of level observed in lakes by our ances- 

 tors or contemporaries ; in a word, to facts which have taken place 

 under the eyes of men, inasmuch as it is the influence of their ag- 

 ricultural labors upon the meteorological state of the atmosphere, 

 which I am seeking to appreciate. The facts upon which I shall 

 more particularly dwell, were observed in South America ; but I 

 shall show that what is true with regard to this continent, is true 

 also with reference to any other continent. 



One of the most interesting portions of Venezuela is, undoubtedly, 

 the valley d'Aragua. Situated at a short distance from the sea- 

 board, possessed of a warm climate, and of a soil fertile beyond ex- 

 ample, it combines within itself all the varieties of agriculture that 

 belong in peculiar to tropical regions ; on the hillocks which rise in 

 the bottom of the valley, are seen fields which bring to mind the 

 agriculture of Europe. Wheat succeeds pretty well upon the heights 

 which surround La Vittoria. Bounded on the north by a chain of 

 hills which run parallel with the sea-board, and to the south by the 

 range which separates it from Llanos, the Aragua Valley is limited 

 on the east and west by a series of lesser elevations, which shut it 

 in completely. In consequence of this peculiar configuration of 

 country, the rivers which rise in its interior have no outlet to the 

 ocean ; their waters accumulate in the lowest part of the valley, and 

 form the beautiful lake Valentia. This lake, which M. de Humboldt 

 says exceeds the lake Neufch^tel in size, is raised about 1300 feet 

 above the level of the sea ; it is about ten leagues in length, and 

 about two leagues and a half where it is widest. 



At the time when M. de Humboldt visited the Aragua Valley, the 

 inhabitants were struck with the gradual diminution which had been 

 going on in the waters of the lake during the last thirty years. J^ 



42* 



