RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 37 



in determining the character and influence of surface temperatures, 

 the telethermoscope offers great possibilities. 



The soil thermograph is desirable for continuous records of soil 

 temperature which can hardly be obtained with any other instru- 

 ment, and particularly for measuring the extremes, which it is almosl 

 impossible to obtain with registering thermometers. These can be 

 had only after careful adjustment-of the range of the pen. or by fre- 

 quent checking against a thermometer, at high and low temperatures, 

 after the instrument is installed. This instrument should at leasl 

 be employed wherever new stations are being established and until 

 the daily curve has been worked out for each season. The securing 

 of a record with this instrument is very similar in its routine fea- 

 tures to the work with air thermographs. There is. however, one 

 feature of the soil thermograph which deserves special consideration. 

 This is the tendency, whenever the instrument is moved and the con- 

 necting tube suffers more or less deformation, for the whole appa- 

 ratus to go through a gradual readjustment. One frequently finds 

 the pen steadily ascending or descending for a week after any change. 

 For this reason it does not appear practicable to calibrate or adjust 

 the recording apparatus to agree with an accurate thermometer be- 

 fore placing the instrument and its bulb in their final positions. The 

 ideological Society, in outlining methods for a systematic soil tem- 

 perature survey, however, recommended calibrating soil thermographs 

 by placing the bulb in a pan of water with the thermometer, and after 

 placing the bulb in its final position in the soil trusting completely 

 to the accuracy of the thermograph. 



It is believed, in view of what has been said, to be absolutely 

 necessary to have a thermometer so placed in a wooden or porcelain 

 tube that its bulb is at the same level and pratically in contact with 

 the bulb of the thermograph, and to obtain frequent comparisons of 

 the thermograph and thermometer. 



Special Suggestions ox Surface Measurements. 



It has been stated that the extremely high temperature attained at 

 the surface of a well-insolated soil seem to have an important heal- 

 ing on the initiation of plants, and that technical difficulties make the 

 actual measurement of this surface temperature almost impossible. 

 Doubtless this could be accomplished from time to time with a super- 

 sensitive plate, such as constitutes a part of the leaf-temperature 

 apparatus, but the problem of recording the maximum attained in 

 a day or a season would still be unsolved. 



It "should be admitted, therefore, that a record of the maximum 

 temperature at the soil's surface can only he approximated with 

 present equipment, for the very simple reason that the object which 



