354 On Rain, Evaporation, Se. 
being multiplied together, will give the quan- 
tity of water that flows down in that time. 
Dr. Halley, in order to estimate the quantity 
of water that flows into the Mediterranean Sea 
by means of rivers, makes a comparison of the 
great rivers of Italy, &c. with that of the 
Thames. (Philos. Transact. Abridg. Vol. 2. 
Page 110). He assumes the breadth of the 
Thames at Kingston Bridge to be 100 yards, 
its depth 3 yards, and velocity 2 miles per hour. 
He professedly overrates the dimensions, in order 
to allow more than a sufficiency for the streams 
_ received below Kingston. This assumption 
gives the area of a transverse section of the 
river==300 square yards, and the quantity of 
water flowing down= 20.300 Ooo tons in a day. 
This must be overrated by at least, I think, one 
third :—If the breadth be assumed 100 yards, 
the depth 3, and velocity 2 miles per hour, it 
will then give 3 of the result above mentioned ; 
or it will amount to the same thing if we take 
< part from all the three data assumed by Dr. 
Halley, the result being 3 of that above; 
amounting in the year to 166.624.128.000 
cubic feet, which is a little more than #5 part 
of all the rain and dew in England and Wales 
ina year, as above deduced, 
By an inspection of the annexed map of the 
rivers of this country, as well as by a fair calcue 
