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 lie adjusted to the seasons. 



In tiie 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 «ase. would make possible the calibration of such an instrument 

 so that the t em perat in v difference between the bulb and the sur- 

 rounding aii- might he directly converted into rate of heat absorp- 

 tion by the bulb. 



\ ;i 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 Hit' 

 regular equipment being in the variable surface exposed to the sun 

 ai different hours by a cylindrical bulb. 



