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

INFLUENCE OF FORESTS
A moment's comparison of the two columns will make the
principle apparent. The temperature of the air falls nearly
fifteen degrees in five days; the temperature of the tree,
sluggishly following, falls in the same time less than four
degrees. Between the 19th and the 20th the temperature of
the air has changed its direction of motion, and risen nearly
a degree; but the temperature of the tree persists in its
former course, and continues to fall nearly three degrees
farther. On the 21st there comes a sudden increase of heat,
a sudden thaw; the temperature of the air rises twenty-five
and a half degrees ; the change at last reaches the tree, but
only raises its temperature by less than three degrees ; and
even two days afterwards, when the air is already twelve
degrees above freezing point, the tree is still half a degree
below it. Take, again, the following case : —
Date.
1859.
July 13,
„ 14,
„ 15,
,, 16,
„ 17,
„ 18,
„ 19,
Temperature
of the Air.
Temperature
in the Tree.
84-92°
76-28°
82-58°
78-62°
80-42°
77-72°
79-88°
78-44°
73-22°
75-92°
68-54°
74-30°
65-66°
70-70°
The same order reappears. From the 13th to the 19th the
temperature of the air steadily falls, while the temperature of
the tree continues apparently to follow the course of previous
variations, and does not really begin to fall, is not really
affected by the ebb of heat, until the 17th, three days at least
after it had been operating in the air. 1 Hence we may conclude
that all variations of the temperature of the air, whatever be
their period, from twenty-four hours up to twelve months, are
followed in the same manner by variations in the temperature
Complex Rendus de I'Academie, 29th March 1869.
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