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 Eecord," 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 tem- 

 peratures 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: 



8 In any ordinary comparison of the temperatures of plant habitats, hourly tempera- 

 tures are not likely to be used except to explain transient phenomena. However, the 

 thermograph is an extremely valuable adjunct in determining the maximum, minimum, 

 and mean temperatures, not only helping to correct errors of observation but making 

 possible the more exact determination of the extremes and temperature ranges for any 

 period, bucIi as the midnight to-midnight day, which is the unit of time in most meteo- 

 rological computations. 



♦Various rules for applying corrections to thermograph traces are used by different 

 students. It is obvious thai errors may exist in the traces from two distinct causes: 

 in When tin- range of oscillation of 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 correel in its possible range there is a Lag due both 

 to tin- lesser sensitiveness of 1 1 1 <- thermograph as compared with a mercurial thermometer 

 and to the friction of the pen upon the paper, so thai normally the pen does not quite 

 reach to the extremes Indicated bj the thermometers, in the flrsl ease, it is essential 

 thai the error be distributed lomewhal according to the temperatures; thus, 11 the pen 

 read correctly al a temperature of IS, at all temperatures above 45 the range of the pen 

 beinx too pjreal there would be a minus correction lor the trace, ami »\ all temperatures 

 ir, there would be a plus correction. On the other hand, it the Instrument Is 

 properly adjusted, it i logical to apply a minus correction to all descending portions of 

 the trace and a plu« correction to all ascending portions, the amount of such correc 

 Hon to he determined from the corrections at the mlnln and maximum, respectively, 



