ON RAIN, EVAPORATION, &C. 171 



cording to the preceding obfervations, the evaporation ought 



to exceed the medium. 



3d. But the principal caufe of the excefs in our account ofa»d the water 



evaporation, I conceive to be the prevention of the water StJ^'nin " 



running off from the furface of the earth at the top, by having freely off at top, 



the earth below the level of the upper pipe : it has been feen, did incr f. afe the 



rr r r evaporation* 



that when the earth was above that level, a great part of the 



water came off that way, by which the furface was fooner 

 dried : whereas by forcing all the water to fink through the 

 earth or (land on its furface, a greater degree of moifture per- 

 petually exifted at the furface, and confequently afforded a 

 greater fcope for evaporation, than the furface of the earth in 

 general would do. 



Upon the whole then I think we may fairly conclude — that Hence the rain 

 the rain and dew of this country are equivalent to the quantity fu Pp tythe eva- 

 of water carried off by evaporation and by the rivers. And as poration and the 

 nature a&s upon general laws, we ought to infer, that it muft nvers * 

 be the cafe in every other country, till the contrary is proved. 



This conclufion being admitted, we are enabled to deduce Theorem for 



a general theorem for the quantity of water carried down into deduc ! ng ^ . 



r • • ' • quantity of wa- 



the fea by any river in any country (on the luppontion that all ter carried by a 



rivers are ramified alike) provided we have certain data : thefe rJvei > &c » 



data are the length of the river, and the excefs of the rain 



above the evaporation in the country from which the water of 



the river is drawn : alfo, it fhould be known by obfervation, 



how much water fome one given river carries down. 



For, from the principles of geometry, the area of country 

 from which any river is fupplied, will be as the fquare of the 

 length of the river ; and the quantity of water carried off, will 

 be in the compound ratio of the area of the country, and the 

 excefs of the rain and dew above the evaporation. 



Thus, let L == the length of any river, E = the excefs of Statement, 

 rain and dew above the evaporation, and Q = the quantity of 

 water difembogued in any given time by that river ; 1 = the 

 length of any other river, e = the excefs, &c. and q = the 



. '■". Qle 



quantity of water ; then we (hall have q = yr*. 



Ex. gr. Suppofe the length of the Thames = 200 miles, Example; 

 and the excefs = 5 inches, eftimating the rain and dew at 30 

 jrich.es and evaporation at 25 ; and fuppofe the river Kent, in 



Weftmorland, 



