On Raia, Evaporation, Ge. 365 
earth or stand on its surface, a greater degree 
of moisture perpetually existed at the surface, 
and consequently afforded a greater scope for 
evaporation, than the surface of the earth in ge. 
neral would do. 
Upon the whole then I think we may fairly 
conclude—that the rain and dew of this country 
are equivalent to the quantity of water carried off 
by evaporation and by the rivers. And as na- 
ture acts upon general laws, we ought to infer, 
that it must be the case in every other country, 
till the.contrary is proved. 
This conclusion being admitted, we are en- 
abled to deduce a general theorem for the quan- 
tity of water carried down into the sea by any 
river in any country (on the supposition that all 
rivers are ramified alike) provided we have 
certain data: these data are the length of the 
river, and the excess of the rain above the eva- 
poration in the country from which the water of 
the river is drawn: Also, it should be known by 
observation; how much water some one given 
river carries down. 
For, from the principles of geometry, the 
area of country from which any river is sup- 
plied, will be as the square 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 excess of the rain and dew 
above the evaporation, 
