278 



climate. Notwithstanding the high tempera- 

 ture of the air, the loss caused by evaporation * 

 will scarcely counterbalance, in deep basins, 

 the advantages of the tropical rains. The ex- 

 periments made at the Pontinspar marshes, by 

 M. de Prony, and at the canal of Languedoc -f, 

 by MM. Pin and Clausade, indicate, in the 

 latitudes 41° and43J°, a produce of annual eva- 

 poration of 348 lines. The experiments which 

 I made in the tropics,, are not sufficiently nu- 

 merous to draw a general result ; but in sup- 

 posing the atmosphere equally calm in the 

 south of France, and the torrid zone, the mean 

 heat of the year to be 15° and 27° cent., and the 

 mean humidity expressed by the degrees of the 

 hair-hygrometer, 82° and^86° I find, with Wl 9 

 Gay-Lussac, that the evaporation of the two 

 zones is in the relation of 1 to 1*6, while the 

 quantity of rain-water which the earth receives, 

 serves as 1 to 5. We must not either forget 

 that canals lose by evaporation only in propor- 

 tion to their own surface, while they collect the 

 waters that fall on the vast extent of surround- 

 ing lands. In the volume of water which hydrau- 

 lic works require, we must distinguish between 

 that which depends on the capacity of the 



* See above, Vol. iv, p. 148. 

 f Ducros Memoir es sur les quantites cVeau qu 1 exigent les 

 canaux de navigation, 1800, No. II. p. 41. 



