28 Eighteenth Report State Entomologist of Minnesota — 1920 



2. Character of hark 



a. Color 

 h. Surface 



c. Structure 



d. Thickness 



3. Air temperature 



4. Air movement 



5. Evaporation from hark surfaces 



6. Proximitv to other radiating or ahsorhing surfaces 



Experimental Methods 



Mercury incul)ator thermometers were used for taking tempera- 

 tures in the camhium. I'hese were inserted in a hole hored into the 

 end of the log in such a way that the thermometer bulb was located 

 just beneath the bark, about four inches from the end of the log. The 

 expanded portion of the thermometer fitted tightly, thus securely clos- 

 ing the hole in the end of the log. In order to minimize the conduction 

 of heat to and from the log the exposed ends of the thermometers were 

 enclosed in caps. These caps were made of short pieces of unbarked 

 white birch with the center bored out to fit the thermometer. It was 

 later found that these precautions for eliminating conduction were not 

 really necessary since comparable results w^ere obtained by the use 

 of the common type of laboratory thermometer unprotected by caps. 

 The conduction of heat w^as more rapid through the bark than through 

 the glass of the thermometer. This might not be true of logs with 

 very heavy scaly bark. 



The intensity of solar radiation was measured by the comparison 

 of a black and a white bulb thermometer, mounted side by side. The 

 difference in temperature registered by these thermometers varied with 

 the intensity of light. The greatest difference observed during the 

 summer was taken arbitrarily as 100 per cent light and the observa- 

 tions in this paper are percentages of this standard. 



Unless otherwise stated the term air temperature as used here 

 refers to the temperature in the open as registered by a white bulb 

 thermometer. 



Five species of logs were used in this series of experiments. 

 These were: 1. White pine, Piniis strobus Linn.; 2. Norway pine, 

 Piniis resinosa Aiton : 3. Jack pine, Pmus hanksiana Lambert; 4. Black 

 spruce, Picea mariana Britton, Sterns and Poggenberg; Balsam fir, 

 Abies balsamca Miller. Each species was placed under four different 



