here was a
difference of more than one degree Fahrenheit between the mean annual
temperatures in favour of the open ground. The mean summer temperature
in the wood was from two to three degrees lower than the mean summer
temperature outside. The mean maxima in the wood were also lower than
those without by a little more than two degrees. Herr La Cour[52] found
the daily range consistently smaller inside the wood than outside. As
far as regards the mean winter temperatures, there is an excess in
favour of the forest, but so trifling in amount as to be unworthy of
much consideration. Libri found that the minimum winter temperatures
were not sensibly lower at Florence, after the Apennines had been
denuded of forest, than they had been before.[53] The disheartening
contradictoriness of his observations on this subject led Herr Rivoli to
the following ingenious and satisfactory comparison.[54] Arranging his
results according to the wind that blew on the day of observation, he
set against each other the variation of the temperature under wood from
that without, and the variation of the temperature of the wind from the
local mean for the month:--

+-------------------------------------------------------------+
| Wind. | N. | N.E.| E. | S.E.| S. | S.W.| W. | N.W.|
| |-----+-----+-----+-----+-----+-----+-----+-----|
|Var. in Wood |+0.60|+0.26|+0.26|+0.04|-0.04|-0.20|+0.16|+0.07|
|Var. in Wind |-0.30|-2.60|-3.30|-1.20|+1.00|+1.30|+1.00|+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.
Becquerel's--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 yearl