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waters of the canal of Laiiguedoc, and the basin 

 of Saint Ferreol, lose every year from 0*758 met. 

 to 0*812 met. or from 336 to 360 lines. Mr. de 

 Prony found nearly similar results in the Pon- 

 tine marshes. The whole of these experiments, 

 made in the latitudes of 41° and 49°, and at 

 10*5° and 16° of mean temperature, indicate a 

 mean evaporation of one line, or one and three 

 tenths, a day. Under the torrid zone, in the 

 West India islands for instance, the effect of 

 evaporation is four times as much, according 

 to Le Gaux, and double according to Cassan. 

 At Cumana in a place where the atmosphere 

 is far more loaded with humidity than in the 

 valley of Aragua, I have often seen evaporate, 

 during twelve hours, in the sun, 8*8 mill., in the 

 shade 3*4 mill. ; and I believe, that the annual 

 produce of evaporation in the rivers near Cu- 

 mana is not below T one hundred and thirty 

 inches. Experiments of this kind are extremely 

 delicate, but what I have related will suffice to 

 demonstrate how great must be the quantity of 

 vapour, that rises from the lake of Valencia, 

 and from the surrounding country, the waters 

 of which flow into the lake. I shall have occa- 

 sion elsewhere to resume this subject ; for, in 

 a work which displays the great laws of nature 

 under different zones, we must endeavour to 

 solve the problem of the mean tension of the 

 vapours contained in the atmosphere in different 



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