118 Proceedings of the Royal Society 
A. moment’s comparison of the two columns will make the prin- 
ciple 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 tempera- 
ture 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. 
Temperature 
of the Air. 
Temperature 
in the Tree. 
July 13, 
8B92 
76-28 
)> 
14, 
82-58 
78-62 
15, 
8042 
77-72 
?> 
16, 
79-88 
78-44 
? ? 
17, 
73-22 
75-92 
>? 
18, 
68-54 
74-30 
55 
19, 
65-66 
70-70 
order 
reappears. 
From the 1 
3th to the 19th the tern- 
perature 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 ope- 
rating in the air.* 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 occur- 
rence than those in the air. This thermal sluggishness , so to speak, 
seems capable of explaining all the phenomena of the case without 
* Comptes Rendus de l’Academie, 29th March 1869. 
