Collected works > Edinburgh edition, 1894-98 - Works of Robert Louis Stevenson > Volume 28, 1898 - Appendix

ON THE THERMAL
of the temperature of the wind from the local mean for the
month : —
Wind, . . .
Var. in Wood,
Var. in Wind,
N.
N.E.
E.
S.E.
S.
s.w.
w.
N.W.
+ 0-60
-0-30
+ 0-26+0-26
-2-60-3-30
+ 0-04
-1-20
-0-04
+ 1-00
-0-20 +0-16
+ 1-30+1-00
+ 0-07
+ 1-00
From this curious comparison, it becomes apparent that the
variations of the difference in question depend upon the amount
of variations of temperature which take place in the free air,
and on the slowness with which such changes are communicated
to the stagnant atmosphere of woods ; in other words, as Herr
Rivoli boldly formulates it, a forest is simply a bad conductor.
But this is precisely the same conclusion as we have already
arrived at with regard to individual trees ; and in Herr Rivoli's
table, what we see is just another case of what we saw in
M. BecquereFs — the different progression of temperatures. It
must be obvious, however, that the thermal condition of a
single tree must be different in many ways from that of a
combination of trees and more or less stagnant air, such as we
call a forest. And accordingly we find, in the case of the
latter, the following new feature : The mean yearly temperature
of woods is lower than the mean yearly temperature of free
air, while they are decidedly colder in summer, and very little,
if at all, warmer in winter. Hence, on the whole, forests are
colder than cleared lands. But this is just what might have
been expected from the amount of evaporation, the continued
descent of cold air, and its stagnation in the close and sunless
crypt of a forest ; and one can only wonder here, as elsewhere,
that the resultant difference is so insignificant and doubtful.
We come now to the third point in question, the thermal
influence of woods upon the air above them. It will be
remembered that we have seen reason to believe their effect to
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