THEORY of RAIN. 69 



WE may next compare the two extents of land which fupply 

 with water thofe two compared evaporating furfaces. If, in 

 thofe two cafes, the furfaces of land, collecting rain, be equal- 

 ly proportioned to the furfaces from which that water is to be 

 evaporated ; and, if we (hall fuppofe, that the quantity of rain 

 which falls upon thofe two furfaces of land, is equal in propor- 

 tion to their fpaces ; in that cafe, we mould conclude, that all 

 the water received into thofe lakes might be evaporated, as it is 

 in the Cafpian fea. If, on the contrary, an immenfe quantity 

 of water i flues annually from thofe lakes, while there is reafon 

 to conclude, from the comparison of the Cafpian, that all the 

 received water fliould be evaporated, we mall then have rea- 

 fon to conclude, that there falls much more rain upon this in- 

 land place of North America, than upon a fimilar fituation in 

 the great continent of Europe and Aiia. The facl: is, that the 

 Cafpian receives its water from a furface of land, larger, in. 

 proportion, than that from whence the lakes of America are 

 fupplied ; confequently, there mould no water ifliie from the 

 lakes, but all evaporate, according to this rule. 



IN this calculation, the evaporation has been fuppofed to be 

 equal, from equal fpaces, on the furface of the Cafpian and the 

 lakes. But it may be alleged, that the evaporation from the 

 lakes may be lefs than that from the Cafpian ; confequently, there 

 might appear to be a redundancy of water, in this cafe, from 

 the lakes, although the proportion of rain might be no greater. 

 This argument, upon another occafion, might be conclufive ; 

 but, in this cafe, where a greater power of evaporating water 

 muft attend the lefTer quantity of condenfed vapour, as flowing 

 neceflarily from the fame caufe, viz. the great diftance from 

 the fea, to allege a greater evaporating power upon the furface 

 of the Cafpian than on that of the lakes, is evidently to give 

 up the argument ; unlefs the greater evaporation fhall be fuppo- 

 fed to arife from a different caufe j and here the only caufe of 



this 



