y
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 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:[55] A and B were both about seventy feet
above the surface of the ground; but A was at the summit of a chestnut
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.[56] It
will be observed that I have omitted up to the present point all mention
of station C. I do so because M. Becquerel's