gainst vertical
rain, and against dew, or what is much the same thing, against the cold
of a clear blue sky on a still night. The temperature of the heavens is
known pretty accurately, by more than one method of calculation: it is
-239 degrees Fahr.; the greatest cold felt in the Arctic regions being
about -40 degrees Fahr. If the night be cloudy, each cloud is a roof to
keep off the cold; if it be clear, we are exposed to the full chill of
the blue sky, with only such alleviation as the warming and the
non-conducting powers of the atmosphere may afford. The effect is greater
than most people would credit. The uppermost layer of the earth, or
whatever may be lying exposed upon it, is called upon to part with a
great quantity of heat. If it so happen that the uppermost layer is of a
non-conducting nature, the heat abstracted from it will be poorly
resupplied by communication from the lower ones. Again, if the night be a
very calm one, there will be no supply of warmth from fresh currents of
air falling down upon it. 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. Hence the value