358 On Rain, Evaporation, E96. 
sent themselves to view, as far as respects €va- 
poration, namely, waler, ground covered with 
grass and other vegetables, and bare soil. The dif- 
ficulties that occur in attempts to find the 
quantity of water evaporated in those three 
cases, are perhaps the principal reason why our 
knowledge on this head is so imperfect. 
As far as experiments hitherto made autho- 
rise us to draw conclusions, it should seem that 
the evaporation from water is greatest ; that from 
green ground is probably next, and that from 
bare soil the least: though we may presume, 
that the copious dews upon the grass more than 
supply the excess of evaporation above -what 
takes place from a moist uncovered soil. 
The most satisfactory experiments I have 
seen an account of, relating to the evaporation 
from a surface of water, are those of Dr. 
Dobson, made at Liverpool, in the years 1772, 
73, 74 and 75. (Vid. Philos. Transac. Vol. 67) 
—He took a cylindrical vessel of 12 inches 
diameter, and having nearly filled it with water, 
exposed it besides his rain-gage of the same 
aperture, and by adding water to it, or taking 
it away occasionally, he kept the surface nearly 
of the same height, and carefully registered the 
quantities added or taken away, by a compa. 
rison of which with the rain, the amount of the 
evaporation was ascertained, ‘The mean monthly 
