Hence, in the treble event of a clear blue sky,
a non-conducting soil, and a perfectly still night, we are liable to have
great cold on the surface of the ground. This is shared by a thin layer
of air that immediately rests upon it; while at each successive inch in
height, the air becomes more nearly of its proper temperature. A vast
number of experiments have been made by Mr. Glaisher on this subject
('Phil. Trans.' 1847), the upshot of which is that a thermometer laid on
grass, under a blue sky on a calm night, marks on an average 8 degrees
Fahr. colder than one 4 feet above it; 1 inch above grass, 5 1/2 degrees;
1 foot, 1 degrees; 4 feet, 1/2 degrees; on gravel and sand the
differences are only about one-third as much. Sheep have a practical
knowledge of these differences. Often, in an early walk on dewy mornings,
I see all the sheep in Hyde Park bivouacked on the gravel walks of Rotten
Row. The above figures are the results of experiments made in England,
where the air is always moist, and the formation of dew, while it
testifies to the cold of the night, assists largely to moderate it. In
arid climates the chill would be far greater; such would also be the case
at high elevations. One of Mr. Glaisher's experiments showed a difference
of no less than 28 degrees between the cold on the ground and that at 8
feet high. This might often be rivalled in an elevated desert, as in that
of Mongolia.
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