PNEUMATICS. 319 



or to supply all the springs, and of consequence all 

 the rivers derived from them, on the surface of the 

 earth. 



Dr HALLEY shewed, that the evaporation from the 

 sea alone is a sufficient supply for all the water 

 that the rivers carry into it. His calculation was 

 founded on a very complex view of the subject, and 

 liable to several objections. BUFFON took a more 

 simple view of the matter, by selecting one of those 

 lakes that sends out no stream to the ocean, and 

 shewing that the probable evaporation from the sur- 

 face of the lake is equal to all the water carried in- 

 to it. 



It may be also satisfactory to shew, that all the wa- 

 ter annually emptied by a river into the sea, is less 

 than the rain, which falls on the surface drained by 

 it. Thus, according to Dr HALLEY'S computa- 

 tion, the water which the Thames carries down 

 through Kingston Bridge, to which the tide does 

 not reach, is 25344000 cubic yards, or 684288000 

 cubic feet per day, which gives 249765120000 cu- 

 bic feet for the quantity of water which the 

 Thames discharges annually into the sea. Now, 

 the surface drained by the Thames and its branches, 

 appears to be about equal to a circle of 40 miles 

 radius; that is, nearly to 5036 square miles, or 

 140395622400 square feet. But if we suppose that 

 the depth of rain which falls annually, at an ave- 

 rage, over this surface, is two feet, we have a quan- 

 tity which exceeds that carried down by the Thames 



by 



