nces very much larger and better marked. To these observations I
proceed at once. Experience has been acquired upon the following three
points:--1, The relation between the temperature of the trunk of a tree
and the temperature of the surrounding atmosphere; 2, The relation
between the temperature of the air under a wood and the temperature of
the air outside; and, 3, The relation between the temperature of the air
above a wood and the temperature of the air above cleared land.

As to the first question, there are several independent series of
observations; and I may remark in passing, what applies to all, that
allowance must be made throughout for some factor of specific heat. The
results were as follows:--The seasonal and monthly means in the tree and
in the air were not sensibly different. The variations in the tree, in
M. Becquerel's own observations, appear as considerably less than a
fourth of those in the atmosphere, and he has calculated, from
observations made at Geneva between 1796 and 1798, that the variations
in the tree were less than a fifth of those in the air; but the tree in
this case, besides being of a different species, was seven or eight
inches thicker than the one experimented on by himself.[48] The
variations in the tree, therefore, are always less than those in the
air, the ratio between the two depending apparently on the thickness of
the tree in question and the rapidity with which the variations followed
upon one another. The times of the maxima, moreover, were widely
different: in the air, the maximum occurs at 2 P.M. in winter, and at 3
P.M. in summer; in the tree, it occurs in winter at 6 P.M., and in
summer between 10 and 11 P.M. At nine in the morning in the month of
June, the temperatures of the tree and of the air had come to an
equilibrium. A similar difference of progression is visible in the
means, which differ most in spring and autumn, and tend to equalise
themselves in winter and in summer. But it appears most strikingly in
the case of variations somewhat longer in period than the daily ranges.
The following temperatures occurred during M. Becquerel's observations
in the Jardin des Plantes:--

Date. Temperature of Temperature in
the Air. the Tree.

1859. Dec. 15, 26.78 deg. 32 deg.
" 16, 19.76 deg. 32 deg.
" 17, 17.78 deg. 31.46 deg.
" 18,