10 



THE UTILITY OF FORESTS. 



both cases. Mathieu started three double stations near Nancy, 

 and Ebermayer seven in various parts of Bavaria. Soon after- 

 wards Switzerland followed with similar stations in the Canton 

 Bern, also Austria, Italy, several other German States, Sweden, 

 Russia and others. The result has been a rich crop of observations. 

 Although many questions await a final solution, much has been 

 learned which will be indicated in the following pages. In order 

 to concentrate the data, they will be given according to seasons 

 as : — 



Spring comprising March, April and May. 



Summer ,, June, July and August. 



Autumn ,, September, October and November. 



Winter ,, December, January and February. 



1. Effect of Forests on Temperature. 



(a) The Teynperature of the Air. — Observations have shown that 

 forests near the 50th degree of northern latitude slightly reduce 

 the mean annual temperature of the air, on an average by about 

 1 degree Fahr. as compared with the temperature on open land, 

 the measurements being taken at 5 feet from the ground in both 

 cases. The difference is less in the crowns of the trees ; it is more 

 in mountainous districts than in the plains. Evergreen forests 

 with a dense leaf canopy have the greatest effect, while deciduous 

 thin-crowned woods exercise httle effect. Small as the decrease 

 is, it has, as will be shown further on, a decided effect upon 

 evaporation and the preservation of moisture. 



Of more importance is the difference during the four seasons of 

 the year. It amounts in spring to 1-24 degrees, in summer to 

 2-54, in autumn to 1-13 and in winter to 0-61 degrees. The 

 observations further show that the minimum temperature at 

 night and the maximum during the day showed the following 

 increases, +; and decreases, — , as compared with those on open 

 ground : — 



