226 
NATURAL HISTORY 
affords drink for three hundred or four hundred sheep, and 
for at least twenty head of large cattle beside. This pond, 
it is true, is overhung with two moderate beeches, that, 
doubtless, at times, afford it much supply; but then we 
haye others as small, that, without the aid of trees, and in 
spite of evaporation from sun and wind, and perpetual con- 
sumption by cattle, yet constantly maintain a moderate 
share of water, without overflowing in the wettest seasons, 
as they would do if supplied by springs. By my journal of 
May, 1775, it appears that the small and even considerable 
ponds in the vales are now dried up, while the small ponds 
on the very tops of hills are but little affected/^ Can this 
difference be accounted for from evaporation alone, which 
certainly is more prevalent in bottoms ? or rather, have 
not thoso elevated pools some unnoticed recruits, which in 
the night-time counterbalance the waste of the day, without 
which, the cattle alone must soon exhaust them ? And 
here it will be necessary to enter more minutely into the 
cause. Dr. Hales, in his Vegetable Statics," advances, 
from experiment, that the moister the earth is, the more 
dew falls on it in a night ; and more than a double quantity 
of dew falls on a surface of water than there does on an 
equal surface of moist earth." Hence we see that water, 
by its coolness, is enabled to assimilate to itself a large 
quantity of moisture nightly, by condensation, and that the 
air, when loaded with fogs and vapours, and even with 
copious dews, can alone advance a considerable and never- 
failing resource. Persons that are much abroad, and travel 
early and late, such as shepherds, fishermen, &c., can tell 
what prodigious fogs prevail in the night on elevated downs, 
even in the hottest parts of summer, and how much the 
surfaces of things are drenched by those swimming va- 
pours, though, to the senses, all the while, little moisture 
seems to fall. 
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