121 
of Edinburgh, Session 1872-73. 
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 diffe- 
rent progression of temperatures. It must he 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 ex- 
pected 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 dif- 
ference is so insiguificant and doubtful. 
We come now to the third point in question, the thermal influ- 
ence 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 cur- 
rent 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 : * 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 differ- 
ences 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 
* Comptes Rendus, 28th May 1860. 
