Collected works > Edinburgh edition, 1894-98 - Works of Robert Louis Stevenson > Volume 28, 1898 - Appendix

ON THE THERMAL
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 him-
self. x 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 pro-
gression 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 varia-
tions somewhat longer in period than the daily ranges. The
following temperatures occurred during M. Becquerel's obser-
vations in the Jardin des Plantes : —
Date.
Temperature
of the Air.
Temperatur*
in the Tree.
1859. Dec. 15, . . 26-78°
32°
, 16,
19-76°
32°
, 17,
17-78°
31-46°
, 18,
13-28°
30-56°
, 19,
12-02°
28-40°
, 20,
12-54°
25-34°
, 21,
38-30°
27-86°
, 22,
43-34°
30-92°
, 23,
44-06°
31-46°
1 Atlas Meteoroloyique de I' Observatoire Imperial, 1867.
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