148 



latitudes, and at different heights above the 

 surface of the ocean. 



A great number of local circumstances cause 

 the produce of evaporation to vary ; it changes 

 as more or less shade covers the basin of the 

 waters, with their state of motion or of repose, 

 with their depth, and the nature and colour of 

 their bottom: but in general evaporation de- 

 pends only on three circumstances, the tem- 

 perature, the tension of the vapours contained 

 in the atmosphere, and the resistance which 

 the air, more or less dense, more or less agi- 

 tated, opposes to the diffusion of vapour. The 

 quantity of water, that evaporates in a given 

 spot, every thing else being equal, is propor- 

 tional to the difference between the quantity of 

 vapour, which the ambient air can contain 

 when saturated, and the quantity, which it 

 actually contains. Hence it follows (as Mr. 

 Daubuisson has already observed, in subjecting 

 my hygrometric observations to calculation), 

 that the evaporation is not so great under the 

 torrid zone, as might be expected from the 

 enormous augmentation of temperature ; be- 

 cause, in those ardent climates, the air is habi- 

 tually very humid. 



Since the increase of agricultural industry in 

 the valleys of Aragua, the little rivers, that run 

 into the lake of Valencia, can no longer be re- 

 garded as real supplies during the six months 



