13.28 deg. 30.56 deg.
" 19, 12.02 deg. 28.40 deg.
" 20, 12.54 deg. 25.34 deg.
" 21, 38.30 deg. 27.86 deg.
" 22, 43.34 deg. 30.92 deg.
" 23, 44.06 deg. 31.46 deg.

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 Temperature of Temperature in
the Air. the Tree.

1859. July 13, 84.92 deg. 76.28 deg.
" 14, 82.58 deg. 78.62 deg.
" 15, 80.42 deg. 77.72 deg.
" 16, 79.88 deg. 78.44 deg.
" 17, 73.22 deg. 75.92 deg.
" 18, 68.54 deg. 74.30 deg.
" 19, 65.66 deg. 70.70 deg.

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.[49] 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 of the tree; and that those in the tree are always less in
amount and considerably slower of occurrence than those in the air. This
_thermal sluggishness_, so to speak, seems capable of explaining all the
phenomena of t