174 WATER AND FOREST, 



that woods reduce temperature and increase the absolute and relative 

 humidity of the air, and, therefore, must to some extent increase pre- 

 cipitation. EFFECTS OF FOLIAGE ON MOISTURE. 



The efficiency of foliage in mechanically arresting and condensing 

 moisture is well known to everyone who has pushed through the brush 

 on a misty morning, or watched the steady drip from the trees during a 

 fog in the lowlands, or during clouds in the mountains. 



A notable instance of this feature of vegetation is seen at Ascen- 

 sion, a volcanic island lying about eight degrees south of the equator 

 and nearly midway between Africa and South America. The island 

 is about 30 to 40 square miles in area. It is used as a naval station by 

 the British government. The water supply is obtained from near the 

 top of Green mountain, at an elevation of about 2,800 feet above sea 

 level. The mountain gets its name from being about the only green 

 spot on the island. Its verdure is maintained by the nearly constant 

 drip from trees and rocks of the moisture mechanically collected from 

 clouds and fogs, and light, passing showers. It is this drip which is 

 chiefly instrumental in maintaining the ground-storage upon which the 

 water supply of the station depends. 



That the drip from trees should play a prominent part in a do- 

 mestic water supply is remarkable testimony to the efficiency of woods 

 in mechanically increasing precipitation. It illustrates beyond any 

 peradventure that trees and brush, in situations of cloud and fog, have a 

 decided value as agents for adding moisture to the soil. Interesting 

 sxperiments have been made as to the amount of concentration of 

 aqueous vapor by leaves, but it is probable that these experiments, 

 nade on detached leaves, did not include the effect of air currents. We 

 know that in evaporation a rapid movement of the air is found to 

 contribute very materially to the rate of evaporation by bringing 

 fresh surfaces into contact. Similarly, air movement must materially 

 augment the rate of mechanical condensation. (In the more modern 

 formula for computing evaporation, wind velocity is always a factor.) 

 Cloud or fog is a manifestation of water in suspension, and it is obvious 

 that the more rapidly the cloud is moved against any surface the more 

 prater will be brought into contact with that surface in a given interval 

 jf time. We all know the wetting through capacity of a mist on a 

 driving wind. Unless the condensation tests were conducted with 

 reference to velocity, they would fail to give full value to the con- 

 densing action. 



