RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 51 
The apparatus is adjusted by increasing or decreasing the amount 
of mercury in the tube and by bringing the tube closer to or farther 
from a vertical position, so that the mercury first reaches the platinum 
wires when the disk of the sun is visible through the clouds. Any 
addition to the light intensity above this approximate standard does 
not, therefore, alter the nature of the record. While the method is 
thus seen to be extremely crude, the record showing only the presence 
or absence of light of a rather low intensity, still it can hardly be 
questioned that such a record of sunlight duration is of very great 
value in comparing the solar climate of different regions, and possibly 
also in obtaining a measure of the direct light under canopies ; that is, 
of the approximate degree of shading. There appears to be an 
untried value in such records through arbitrary rating of the recorded 
" sunshine '' according to the elevation of the sun, and with allowance 
also for atmospheric humidity- 
One objectionable feature of the instrument is the amount of time 
required to warm it in the morning to the point where it first records. 
In fact, it is by no means free from effect of the air temperature and 
must be adjusted to the seasons. 
In the lack of a better measure of sunlight values, it seems well 
worth while to have this sunshine record in forest studies. The form 
for " Daily and Hourly Sunshine Duration " has been provided for 
the tabulations of a month. 
6. The solar thermograph or mechanical differential telethermo- 
graph devised by Briggs (54) in the biophysical laboratory of the 
Bureau of Plant Industry, is in principle the same as some of the 
soil thermographs ; but it is a duplex instrument, in which the tem- 
perature of one of the bulbs tends to compensate that of a second. 
One of the bulbs may be blackened and spherical, with a short tube, 
so that the bulb is rather easily held just above the case of the in- 
strument, while the second bulb may be kept in the shade. 
This arrangement permits the recording of the excess of tempera- 
ture attained by the bulb in sunlight, limited by the natural radia- 
tion and by conduction, which will increase as the air movement in- 
creases. The reduction of air movement to practically zero, or the 
elimination of conduction almost entirely by the use of an evacuated 
glass case, would make possible the calibration of such an instrument 
so that the temperature difference between the bulb and the sur- 
rounding air might be directly converted into rate of heat absorp- 
tion by the bulb. 
As a matter of fact, an ordinary air and soil thermograph has 
been used by Bates (105) with a fair degree of satisfaction, to show 
the variations in sun heat from day to day, the disadvantage of the 
regular equipment being in the variable surface exposed to the sun 
at different hours by a cylindrical bulb. 
