366 On Rain, Evaporation, &e. 
Thus, let L=the length of any river, E=2 
the excess of rain and dew above the evaporation, 
and O=the quantity of water disembogued ir 
any given time by that river; l—the length of 
any other river, e=the excess, &c. and q=the 
quantity of water; then we shall have q— 
Ole 
LE 
Ex. gr. Suppose the length of the Thames 
=200 miles, and the excess—5 inches, estima- 
ting the rain and dew at 30 inches and evapor- 
ation at 25; and suppose the river Kent, in 
Westmorland, to be 20 miles in length, and the 
excess 35 inches, the rain and dew being sup- 
posed 65, and evaporation g0 inches. 
BO: a ee want Sa 
Then, 
200° X 5 100 
or O=143q ; which result, I believe, will be 
found to accord nearly with the measurement of 
the two rivers on the principle before men= 
tioned. 
