' Can this
difference be accounted for from evaporation alone, which
certainly is more prevalent in bottoms ? or rather have not those
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, etc., 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 vapours, though, to the senses, all the
while, little moisture seems to fall.
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