On Rain, Evaporation, &e. 855 
jation, it appears, that the water of the Thames 
is drawn from an extent of country of about 600 
square miles, or 3 of the area of the whole, 
nearly. The Severn, including the Wye, 
spreads over an equal or greater extent of 
country: And that collection of rivers which 
constitutes the Humber is superior to either of the 
other two in this respect. As far as my own obser- 
vation goes, the Severn and Wye must disem- 
bogue as. much or more water than the Thames ; 
the. Humber. I have not seen collectedly, but 
have noticed most of the branches constituting 
it, andshould apprehend it can not be inferior 
to the Thames: All other circumstances being 
the same, the quantity of water carried down by 
any river should be as the area of the ground 
from which the water is derived, and on this 
account the Humber ought to exceed the 
Thames.* 
The Severn, which is partly derived from 
the mountainous country of Wales, is certainly 
the most rapid of the three rivers, and probably 
carries down the most water: As the Thames, 
- however, is generally considered to take the 
lead, we will suppose, upon the whole, that 
these three rivers are equal in this respect. 
The counties of Kent, Sussex, Hampshire, 
* A more perfect theorem will be given afterwards, 
for finding the quantity of water carried down by any 
river, 
