It is constantly remarked, that a very slight
mound or ridge will shelter the ground for many feet behind it; and an
old campaigner will accept such shelter gladly, notwithstanding the
apparent insignificances of its cause.
Shelter from the Sky.--The shelter of a wall is only sufficient against
wind or driving rain; we require a roof to shield us against 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.
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