RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 29 



Granting that a full and satisfactory record of soil temperatures 

 may be obtained by the use of the thermograph, it may still, because 

 of the cost of this instrument, be impossible to obtain the desired 

 comparison of a number of sites. The best alternative would seem 

 to be to make one thermograph serve for a number of stations by 

 placing it successively at the several stations until the nature of the 

 diurnal oscillation, for a given season, has been worked out for each 

 station. These oscillations will depend so greatly on the character 

 of the insolation, that a curve for one point could hardly be expected 

 to apply at any other point. With a mean daily curve, however, a 

 single thermometer reading each day may give a very good basis 

 for approximating the mean soil temperature for the day. If this 

 is convenient, the reading may be timed to accord with the most 

 probable hour for the mean temperature to occur. 



With hourly soil temperatures for a period of a week at any 

 season, tabulated on the " Hourly (Air, Soil, or Actinograph) Tem- 

 peratures " form, the mean hourly temperatures may be computed, 

 as well as the mean for all of the days concerned. From the former 

 may be obtained a correction factor for any hour, which, added to 

 the reading for a similar observation hour will give approximately 

 the mean temperature. 



For instance, a study of station A-l at Wagon Wheel Gap, Colo, 

 (steep northerly exposure), at midsummer, showed that the daily 

 oscillation was about 1.35°, that the mean temperature was ap- 

 proached very closely at 7 a. m. or 7 p. m., the minimum not occur- 

 ring until 2 p. m., and that the correction for a 9 a. m. reading, on 

 6 days, varied from -f-0.10° to -(-0.50°, with a mean correction of 

 0.34. Similarly at station A-2 (south exposure), it was found, that 

 the a proximate mean would be read at 5 a. m. or 4 p. m., that the 

 minimum occurred at noon, that the daily oscillation was 2.37°, and 

 that a 9 a. m. reading must be corrected by —0.88° to give the mean 

 for the day. Corrections for six individual days varied from —0.50° 

 to - 1.35 . 



Mooic (11 ) states that at a depth of 3 feet daily oscillations are 

 not felt. It is believed that they are, as a rule, too small even at 

 2 feet to warrant consideration, although in excessively insolated 

 soils the procedure described for 1-foot temperatures may be fol- 

 lowed. 



\ oother method which suggests itself for determining the probable 



variation from t he. men n of any daily temperature reading at a fixed 



hour is to compare (Ik- annual mean temperature at the shallow' 



depth with the mean for I feel or greater depth where, it may be 



imed, the daily values are not affected by regular oscillations. 



For, while at any time the deeper soil may he cooler or wanner than 



the 3urface, the deeper soil always evincing a definite "lag 11 when 



