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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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