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

INFLUENCE OF FORESTS
be similar to that of certain other surfaces, except in so far as
it may be altered, in the case of the forest, by the greater
extent of effective radiating area, and by the possibility of
generating a descending cold current as well as an ascending
hot one. M. Becquerel is (so far as I can learn) the only
observer who has taken up the elucidation of this subject. He
placed his thermometers at three points : x A and B were both
about seventy feet above the surface of the ground ; but A was
at the summit of a chesnut tree, while B was in the free air,
fifty feet away from the other. C was four or five feet above
the ground, with a northern exposure ; there was also a
fourth station to the south, at the same level as this last, but
its readings are very seldom referred to. After several years
of observation, the mean temperature at A was found to be
between one and two degrees higher than that at B. The
order of progression of differences is as instructive here as in
the two former investigations. The maximum difference in
favour of station A occurred between three and five in the
afternoon, later or sooner according as there had been more or
less sunshine, and ranged sometimes as high as seven degrees.
After this the difference kept declining until sunrise, when
there was often a difference of a degree, or a degree and a half,
upon the other side. On cloudy days the difference tended to
a minimum. During a rainy month of April, for example, the
difference in favour of station A was less than half a degree ;
the first fifteen days of May following, however, were sunny,
and the difference rose to more than a degree and a half. 2 It
will be observed that I have omitted up to the present point
all mention of station C. I do so because M. BecquerePs
language leaves it doubtful whether the observations made at
this station are logically comparable with those made at the
other two. If the end in view were to compare the progression
of temperatures above the earth, above a tree, and in free air,
1 Comptes Rendus, 28th May 1860. 2 Ibid., 20th May 1861.
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