RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 19 



When readings are taken in the morning, if there is no thermo- 

 graph record by which the time of the maximum and minimum may 

 be determined, the minimum then read should be tabulated on the 

 form for "Air Temperature Record," as of the current day, and the 

 maximum as of the preceding day. If readings are made in the 

 afternoon, both maximum and minimum should be credited to the 

 current day. The current temperature should, of course, be credited 

 to the day on which taken. The instrumental corrections should be 

 used when entering the data in the field, if cards therefor have been 

 prepared, the card being tacked in a conspicuous place in the instru- 

 ment shelter. 



The daily range — purely a computed quantity — in degrees and 

 tenths, should be the difference between maximum and minimum 

 temperatures as tabulated for any calendar day. 



Hourly Temperatures. 



Where a thermograph is available, the instrument should be set 

 in the same shelter as the maximum and minimum thermometers, and 

 hourly temperatures may be obtained therefrom. 3 Corrections for 

 the thermograph trace should always be obtained from the readings 

 of the maximum and minimum thermometer, as thermograph records 

 are liable to considerable errors; but the hours to which these correc- 

 tions are applied may well be a matter of judgment with the ob- 

 server, depending on the shape of the temperature curve. 4 The tabu- 

 lation of hourly temperatures when obtained will require the special 

 form, " Hourly (Air, Soil, or Actinograph) Temperatures.' Certain 

 data therefrom will be entered on the "Air Temperature Record. ' 

 For example, as a measure of conditions affecting growth rate, it may 

 be desirable to know, besides the mean: 



* In any ordinary comparison of the temperatures of plant habitats, hourly temperatures are not likely 

 to be used except to explain transient phenomena. However, the thermograph is an exl remely valuable 

 adjunct in determining the maximum, minimum, and mean temperatures, not only helping to correct 

 errors of observation butmaking possible the more exact determination of the extremes and temperature 

 ranges for any period, such as the midnight-to-midnight day, which is the unit of time in most meteoro- 

 logical computations. . 



^ Various rules for applying corrections to thermograph traces are used by different students. 

 obvious that errorsmay exist in the traces from two distinct causes: (1) When the range of oscillation ol 

 the pen is too great or too small the thermograph may read correctly at medium temperatures bul be high 

 and low at the two extremes; (2) even if the pen is approximately correct in its possible range there i 

 due both to the lesser sensitiveness of the thermograph as compared with a mercurial thermomet er and to the 

 friction of the pen upon the paper, so that normally the pen does not quite reach I o I he exl remes indicated 

 by the thermometers. In the first case, it is essential that the error be distributed some* h ding 



to the temperatures; thus, if the pen read correctly at a temperature of 45, at all temperatures ftbo> 

 the range of the pen being too great-there would be a minus correction for the trace, and al all tempera- 

 tures below 45 there would be a plus correction. On the other hand, if the instrumenl is , «q ,erly adjusted 

 it is logical to apply a minus correction to all descending portions of the trace and a plus correel ton to all 

 ascending portions, the amount of such corrections to be determined from the corrections at . he minimum 

 and maximum, respectively. 



