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[tem]perature of the atmosphere; being here about 48 degrees of
Fahrenheit's thermometer. A body of water, such as a lake of
considerable depth, forms a kind of mean between the subjacent
earth and the superincumbent air: it surface is influenced by the
temperature of the atmosphere, while its lower parts admit of
less variation; consequently the surface will in summer be the
warmest, and in winter the coldest part. So long as the surface
of water retains its fluidity, it helps to meliorate the
temperature of the air in its vicinity; and its surface being
frozen, the water contiguous to the ice will always be nearly
32°; at the same time the temperature towards the bottom may
be some degrees higher.
In clear weather, the surface both of the earth and of water is
warmed in the day and cooled during the night; but in very
different proportions - the water retaining its heat much longer
than the land. It will sometimes happen in an autumnal evening,
that the temperature of the air and that of the water of the lake
will be equal; and yet before sunrise there will be a difference
of twenty degrees or upwards: in this case the air above the
water being warmer, will contain more vapour than that above the
land, and on their intermixture a mist or fog will be formed;
which will continue to float in the atmosphere till it be either
dissolved by an increase of heat, or being moved into a colder
region, be deposited in the form of dew or hoar frost. Sir
Humphry Davy has observed, that upon some
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