40 GROWTH IN TREES. 
After the corky bark and the softer bast had been removed, leaving 
the woody surface bare for about 5 sq. cm. in two places, to give bearings 
for the contact screw and lever arm, the daily equalizing variation in 
diameter for a week had an amplitude of 0.2 mm. or one part in 1,750 
equivalent to a change of 400 cu. cm. in volume. At the end of this 
time the layers of wood formed in 1920 and 1918 were removed, giving 
a new reduced diameter of 30 cm. After the bearings were adjusted 
on these sunken surfaces, the daily variation (which at first was prac- 
tically equivalent to that of the preceding setting of the whole trunk) 
decreased until August, when the range of variation was about 1 in 
8,750 parts, a range which was maintained with interruptions until 
the close of the observations in October. The variation in the volume 
of the wood amounted to about 80 cu. em. (fig. 16). 
In comparison with the above, it was noted that the relative range 
of the daily equalizing variations in Pine No. 6 decreased in the same 
ratio, both reaching an approximate minimum about September 20. 
The behavior of the two diverged after this date. The dendrographic 
record of the entire trunk of No. 6 shows an increase in the range of 
daily variation, to which was added a positive increase accompanying 
a rain on October 6, which continued until the 12th, when only the 
equalizing variations continued, but these were of an amplitude about 
twice that of the minimum. The woody cylinder of No. 7 interior to 
the layer of 1919 also showed a swelling of the same duration as in 
No. 6, but the range of daily variation did not increase. 
Thermometers were inserted in the trunk of No. 7 on June 1, one 
to take the temperature of the bast and cambium region and a second 
with its bulb at a distance of 12 cm. from the surface, which would 
be within less than 8 cm. of the center. The course of the tempera- 
tures during the day and with the advance of the seasons may be 
illustrated by the following data: At 8 a. m. the outer thermometer 
read 12° C. and the inner 13° C., at 6 p. m. the readings were identical. 
On July 2 the outer temperature at 7 a. m. was 14° C., and the inner 
15° C.; at 3 p. m. the readings were 17° C. and 15.5° C., and at 6 p. m. 
identical readings of 16° C. were taken. On August 8 the outer 
temperature was 12° C. at 7° 30" a. m., and the inner 13° C. At 
1* 30" p. m. the outer temperature had advanced to 15° C. and the 
inner to 13.5° C. The range of temperature in the tree taken by the 
two thermometers was 8° C. during the summer, and the difference 
between the outer and inner readings reached its maximum of 5° C. 
in mid-afternoon, or earlier on still hot days, equalization of the two 
being reached shortly after sunset, with the outer portion of the tree 
becoming cooler during the night, so that at sunrise as much of a dif- 
ference as 1.5° C. might be seen. 
The diameter, therefore, had its minimum measurement at the time 
when the outer layers of the trunk have a maximum temperature. 
