81 



that in a ravine, 13.3 feet (4 m.) below, it was 80 per cent of that 10 

 inches above the surface. The relative seasonal activity from May to 

 November is shown in Figure 11. This diagram shows that after the 

 leaves appear the highest evaporation takes place in July. This is 

 probably the critical season for some animals. 



f ^^^^•. ^h Diagram showing the average daily rate of evaporation in beech-maple 

 trn-est, SIX feet above soil (a), near the surface of soil (b), and in a ravine (c). 

 (From Fuller.) ^ ' 



In the forest, Libernau (Harrington, '93 : 34) found that the "rela- 

 tive humidity increases and decreases with the absolute humidity, 

 whereas it is known in general, and also at the Station in the open 

 country, that these two climatic elements are inverse. This is ac- 

 counted for by the fact that the forest is a source of atmospheric 

 aqueous vapor as well as of cooling." (L. c. : 104: *'The absolute 

 humidity decreases in the forest from the soil upwards. The rate of 

 decrease is usually the greatest under the trees and the least at the level 

 of the foliage. The rate above the trees is intermediate between the 

 other two. This rate is least in the late hours of the night, when it 

 rnay be zero. It increases with the increase of the temperature of the 

 air, becoming greatest in the midday hours, when, under exception- 

 ally favorable circumstances, it may make a difference of 10 per cent. 



