62 A PRIMER OF FOHESTRY. 



powerful influence on these differences, which are also 

 greatly affected by the kind of trees and the density of 

 the forest. 



It must be borne distinctly in mind that the figures 

 given above are reliable only for the places in central 

 Europe where they were observed. But the principles 

 on which they depend are just as true in America as 

 they are in Europe. Natural laws are the same the 

 world over. It is safe to conclude, then, that in the 

 United States the forest modifies the temperature of 

 the air in certain ways and for certain reasons, both of 

 which we have seen. Just how great this influence is 

 in different parts of this continent it is as yet impos- 

 sible to tell. But it is probably greater on the average 

 than these observations indicate, for two reasons: 

 First, the extremes of heat and cold, moisture and 

 dryness, are much greater here than in central Europe, 

 and changes are more sudden; second, in most of the 

 double stations mentioned above the station outside the 

 forest Avas within less than a mile of it, and thus likely 

 to be influenced by the cooler air currents flowing from 

 it; that is, the real effect of the presence or absence of 

 woods over large stretches of country is probably 

 greater than these observations show. 



A system introduced in Austria is expected to give 

 a clearer idea of the distance to which the forest influ- 

 ence reaches. It consists of lines of stations beginning 

 in the center of a large forest and extending step by 

 step into the open county beyond. 



MOISTURE IN FOREST AIR. 



The moisture of the air is greater in the forest than 

 outside. The absolute quantit} r of water vapor in a 



