THE EARTH. l6g 



On the other side of the question* it is assert- 

 ed, that the vapours which are exhaled from the 

 sea, and driven by the winds upon land, are more 

 than sufficient to supply not only plants with mois- 

 ture, but also to furnish a sufficiency of water to 

 the greatest rivers. For this purpose, an estimate 

 has been made of the quantity of water emptied 

 at the mouths of the greatest rivers, and of the 

 quantity also raised from the sea by evaporation ; 

 and it has been found, that the latter by far ex- 

 ceeds the former. This calculation was made by 

 M. Mariotte. By him it was found, upon receiv^ 

 ing such rain as fell in a year in a proper vessel 

 fitted for that purpose, that, one year with another, 

 there might fall about twenty inches of water upon 

 the surface of the earth throughout Europe. It 

 was also computed, that the river Seine, from its 

 source to the city of Paris, might cover an extent 

 of ground, that would supply it annually with 

 above seven billions of cubic feet of this water, 

 formed by evaporation. But, upon computing 

 the quantity which passed through the arches of 

 one of its bridges in a year, it was found to amount 

 only to two hundred and eighty millions of cubic 

 feet, which is not above the sixth part of the former 

 number. Hence, therefore, it appears, that this 

 river may receive a supply brought to it by the 

 evaporated waters of the sea, six times greater 

 than what it gives back to the sea by its current ; 

 and, therefore, evaporation is more than sufficient 

 for maintaining the greatest rivers, and supplying 

 the purposes also of vegetation. 



* Phil. Trans, vol. ii. p. 128. 



