14 FOREST POLICY. 



humus is antagonistic to pathogenic bacteria. So far, no microbes 

 have ever been found in forest soil; a gramm of soil in the field 

 contains from two to three hundred thousand bacteria. 



The absence of dust in the 'forest is explained by the fact that 

 there is less wind in the forest, and that the carpet on the ground 

 does not allow of dust formation. Bacteria are not carried about by 

 the air, but by the particles of dust suspended in the air. 



As a sanitary filter of drinking water, the forest, or rather the 

 soil under the forest, is unexcelled. 



(XVI.) TEMPERATURE. 



The air temperature is certainly more influenced by altitude, lati- 

 tude and proximity of ocean or gulf stream than by the proximity of 

 the forest. 



Daily temperature: The forest air is warmer during the night 

 and cooler during the day than the field air. 



Mean temperature: In summer the forest air is three degrees 

 cooler than field air; in winter there is scarcely any difference. 

 Shade-bearing species show a greater influence than light-demanding 

 species. 



Extreme temperature: The extremes of ternperature seem to 

 be Considerably influenced by the forest, if publications by Woeickof 

 are correct. After him, the forest lessens the extremes of temperature 

 by about ten degrees. For Germany, the summer extremes in the 

 forests are about seven degrees lower than in the field, the winter 

 extremes about two degrees higher. The radial posts from meteor- 

 ological stations now established in Europe may throw additional 

 light upon the subject. 



Soil temperature is limping behind air temperature by about two 

 months, the difference increasing with increasing depth. (In the soil 

 during May the temperature decreases with depth, in October it 

 increases.) 



The soil temperature in the field is almost equal to the air tem- 

 perature in the field. In the forest, there is a difference of two degrees 

 in favor of the air. 



The daily range of temperature in the forest soil is less than in 

 field soil. The summer temperature in field soil is about six degrees 

 higher tl^an in forest soil. In winter there is almost no difference. 



The temperature of a tree bole ranges between soil and air tem- 

 perature. As the temperature of the inner layers of the bole during 

 winter is higher than that of the outer layers, frost cracks are apt to 

 form. 



(XVII.) HUMIDITY. 



The humidity of the air depends more on the altitude and the 

 proximity of the ocean than on the presence or absence of forests. 



The absolute humidity of forest air, all the year around, is equal 

 to the absolue humidity of field air. The relative humidity (propor- 

 tion of absolute to possible humidity), however, is less in the field 



