1881 .] 
297 
[Penhallow. 
power, while yet again the wood will show very different rates 
of conduction according to whether it be with reference to the 
longitudinal or transverse section. Knoblauch has shown the 
conductivity of wood in the longitudinal direction as compared 
with the transverse to be, in acacia, box and cypress as 1.25 : 1 ; 
lime, alder and pine 1.80 : 1 ; elm, oak, beech and walnut 1.45 : 1 ; 
apricot 1.60 : 1. 
As a result of what has just been stated, we find that all parts 
of a tree do not have the same temperature. Also, as our 
figures show, that the temperature of a tree, while it constantly 
tends to approach that of the air, will rarely coincide with it, 
but follows by a varying number of degrees, dependent upon 
the rapidity of atmospheric changes and the degree to which 
they extend. During the winter, when the atmospheric changes 
are greatest, we should doubtless find these differences to be 
greatest, but that they would gradually lessen with the approach of 
warm weather. It also follows that, as the tree absorbs heat and 
radiates it more slowly than the changes occur in the air, radia- 
tion and absorption will continue for a time even after the min- 
imum and maximum temperatures of the air have been passed. 1 
Thus, taking the hours of day and night separately, it is found 
that in either of these periods, while the line of tree temperature 
may show but slight variations, the line of temperature of the 
air will frequently cross it, as will be seen from an inspection of 
the chart. While these variations are hourly, there is also found 
to be a daily variation dependent upon difference of temperature 
between night and day, and as our figures show, the maximum 
temperature for the whole twenty-four hours is not reached until 
sometime after sunset. According to Becquerel this occurs 
about nine o’clock, in the summer months. The minimum is 
reached sometime after sunrise, probably about eight o’clock. 
A further examination of the relation between the temperature 
of air and tree shows that the average temperature of the oak was 
1 As these temperatures were all obtained on the south side cf the tree, it might be 
urged that the results are different from what they would be if taken from some other 
side, but ithas been found that whether athermometer be inserted on the north or south 
side, the readings will be the same. The temperature of the tree depends upon the gen- 
eral temperature of the air and not upon exposure to sunlight. 
