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that when air, fully saturated with vapour, suffers a diminution
of its heat, the water is exhibited in the form of mists, clouds,
dew, or rain. It has been stated by the late Dr. Hutton of
Edinburgh, and more fully exemplified by Dr. Dalton, that the
quantity of vapour capable of entering into air, increases in a
greater ratio than the temperature; therefore, whenever two
volumes of air, of different temperatures, are mixed together,
(each being previously saturated with vapour,) the mean
temperature is not able to support the mean quantity of vapour;
consequently its precipitation in the form of clouds and rain, is
occasioned, not by mere cold, but by a mixture of comparatively
cold and warm air: and on this principle, may be explained many
of the phenomena of mist or fog, clouds, dew and rain.
Different portions of the earth's surface, and of course the
contiguous portions of air, are differently heated by the sun's
rays impinging upon them in various degrees of obliquity; and
this difference is naturally much greater in a mountainous than
in a champaign country; and on two portions of air thus unequally
heated, being intermixed one with the other - either by the
ascent of the warmer and lighter part, or by a gentle current of
the wind - the vapour assumes a visible form.
The temperature of the earth, from a few yards below the surface,
to the greatest depth hitherto explored, suffers little variation
between summer and winter. It corresponds nearly with the mean
tem-
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