32 Eighteenth Report State Entomologist of Minnesota — 1920 



On 15 per cent of the days the temperature under the rough bark ot 

 white pine exceeded 48 degrees C. Under norway pine bark 2 mm. 

 thick, the temperature did not exceed 44 degrees C, while under bark 

 10 mm. thick the temperature did not exceed 35 degrees C. July and 

 August were much dryer, and during these months on 62 per cent of 

 the days the temperature in rough barked white pine exceeded 48 

 degrees C. Thin-barked norway pine reached 48 degrees C. a few 

 times, but norway pine with bark 10 mm. thick at no time exceeded 

 46 degrees C. Except in the early morning it was very iniusual to 

 find the temperature under the bark lower than that of the surrounding 

 air. 



Influence of the Position of the Log with Reference 

 TO THE Sun's Rays 



The temperatures beneath the bark vary greatly in similar logs 

 lying in different positions. A log standing on end will never become 

 heated to as high a temperature as a similar log lying on the ground. 

 Also the action of air currents coupled with the fact that the sur- 

 face of the log is at a distance from other radiating or absorbing sur- 

 faces causes the subcortical temperature of the standing log to be 

 very variable, showing decided fluctuations over short periods of time. 

 (See Plate I. ) The log lying on the ground does not show these fluctu- 

 ations in temperature to any marked degree, but the temperature rises 

 and falls evenly. 



In a log lying in a north and south direction a much larger por- 

 tion reaches a high temperature on a bright day than in a log lying 

 east and west. This is due to the fact that the direct rays of the sun 

 strike only a comparatively narrow strip along the south side of the 

 log lying east and west, while the log lying north and south is heated 

 first on one side and then on the other as the sun moves across the 

 sky. Thus almost half of the log lying north and south may pass 

 above the fatal temperature for insects, while only a quarter or even 

 less of the log lying east and west will reach this temperature. As a 

 rule, however, the comparatively narrow strip on the log lying east 

 and west may reach- a higher temperature than any part of the log 

 lying north and south, owing to the greater length of time that each 

 point on this strip is exposed to direct sunlight. (See Plates II 

 and III.) 



The rate of conduction of heat around the log varies somewhat 

 with the (lifTerent species and is apparently partially correlated with 



