74 



FOREST INFLUENCES. 



tor of lieat. The wood itself is a bad conductor; the tissue is filled 

 witli air or sap, both bad eouductors; and it is protected by a layer of 

 bark Avhich is an especially poor conductor. All these would tend to 

 prevent the access of beat to the interior of the tree and to prevent its 

 exit when there. 



These conditions would explain the retardation of the maximum in 

 trees but would not explain the lower mean tem[)erature nor relatively 

 low temperature of winter. 8ome lij^ht may be thrown on their con- 

 dition l)y the values of the differences of temperature between tree 

 and air surrounding it [T—W). These observations were not made in 

 the German service but Avere in tlie Bavarian. It is necessary, there- 

 fore, to depend here on the nuich shorter series of published observa- 

 tions of the latter service. Observations of the temperature of the tree 

 were taken at the base and in the crown. The thernuuneter was al- 

 ways iutro<luced to the heart. For the stations at the l>ase this dis- 

 tance varied from 4 inches to 11.2 inches averaging 6.3 Paris inches 



In tree crown. Average depth 4.3 inches. In tree base. Average depth 6.3 inches. 



Fig. 44.— Tenii)eratuie diflVreuces between tree aufl air. 



(170 mm.). The observations iu the crown were made at an average 

 height of 31.4 feet (varying from 20 to 40). The depth of fhe ther- 

 mometers varied from 2.7 inches (73 mm.), to 8.5 inches (230 mm.), aver- 

 aging 4.3 Paris inches (110 mm.). The number of trees on which ob- 

 servations were made through the year was eight— five evergreen, and 

 three deciduous. The results are represented in Fig. 44, the broken 

 line showing the tree crown observations, and the dotted line those 

 near the base. The straight lines, parallel to the zero line, represent 

 the corresponding mean values. While in Fig. 43 the mean tempera- 

 tures are used, in Fig. 44 the monthly values of T—Wave shown. 



In the arrangement of each one of the curved lines in Fig. 44 about 

 its mean value, the corresponding straight line, there is nothing but 

 what might be expected in observations iu the interior of any inorganic 

 mass, such as a rock, a mass of masonry, or a dead tree. In fact, 

 except for certain irregularities which are probably due to the shortness 

 of the series of observations, the curves are much like the value of 



