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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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