158 OBSERVATIONS ON THE FALL OF RAIN. 
surface of the earth is continually supplied with 
water,” and after alluding to the importance of 
the enquiry to agriculture, and to the arts and 
manufactures, he enters into a very careful inves- 
tigation of the whole subject, under the several 
heads of— 
“The quantity of Rain and Dew. 
‘‘ The quantity of water that flows into the sea. 
“ The quantity of water raised by evaporation. 
and 
‘‘ The origin of springs.” 
Having first determined, from an extensive 
series of observations, the probable average quan- 
tity of rain falling annually throughout the king- 
dom, which, taking 31 ins. for rain, and 5 ins. 
for dew, he considers will be 36 ins., and by 
tmultiplying this quantity into the area of the 
‘country, ascertained the whole amount of rain, 
he proceeds to estimate how much will be carried 
down by the rivers to the sea’ Grounding his 
calculations upon an estimate made by Dr. Hal- 
ley, upon the volume of the Thames at Kingston 
Bridge, and the extent of country from which 
the supply of water would be drawn, he goes 
through a consideration of the quantity of water 
which must be conveyed to the sea by all the 
