122 Proceedings of the Royal Society 
had been more or less sunshine, and ranged sometimes as high as 
seven degrees. After this the difference kept declining until sun- 
rise, 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 ex- 
ample, 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.* 
It will he observed that I have omitted up to the present point all 
mention of station C. I do so because M. Becquerel’s language 
leaves it doubtful whether the observations made at thist 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, removed from all such radiative 
and absorptive influences, it is plain that all three should have been 
equally exposed to the sun or kept equally in shadow. As the 
observations were made, they give us no notion of the relative action 
of earth-surface and forest-surface upon the temperature of the con- 
tiguous atmosphere ; and this, as it seems to me, was just the crux of 
the problem. So far, however, as they go, they seem to justify the 
view that all these actions are the same in kind, however they may 
differ in degree. We find the forest heating the air during the 
day, and heating it more or less according as there has been more 
or less sunshine for it to absorb, and we find it also chilling it dur- 
ing the night ; both of which are actions common to any radiating 
surface, and would be produced, if with differences of amount and 
time, by any other such surface raised to the mean level of the ex- 
posed foliage. 
To recapitulate : 
ls£, We find that single trees appear to act simply as bad con- 
ductors. 
2 d, We find that woods, regarded as solids, are, on the whole, 
slightly lower in temperature than the free air which they have 
displaced, and that they tend slowly to adapt themselves to the 
various thermal changes that take place without them. 
3 d, We find forests regarded as surfaces acting like any other 
* Comptes Rend us, 20th May 1861. 
