102 



FOREST INFLUENCES. 



It appears lliat tlie forest is of medium activity in tins regard. It is 

 usually surpassed by the ordinary forage crops, but surpasses ordinary 

 fallow growths and far surpasses the average bare soil. 



Still, the forest exercises a strong, conservative effect on the waters 

 within and under it. It saves the most of its ground water from evap- 

 oration, and a great deal of the water above the surface. It differs 

 from other forms of surface covering in drawing its water from a 

 greater depth. It therefore does not dry out the surface so much, 

 and it is also less sensible to temporary droughts. Although less effec- 

 tive, on the average, in adding vapor to the air than are nmny less 

 herbaceous forms of vegetation, it is more steady in its action, and it 

 therefore adds its moisture when it is most needed. During con- 

 tinued dry weather the roots of the grasses dry out the surface soil 

 and exhaust its water. They then cease pouring moisture into the air. 

 At such times the forest may continue its transpiration and at the 

 same time, bj^ its preservative action, its soil is charged with moisture 

 and may continue to feed the springs tributary to it. 



FORESTS AND HUMIDITY. 



The psychrometer observations, within and without forests, permit a 

 study of absolute and relative humidity. The comparative results for 

 the absolute humidity are given in the accompanying table in the form 

 of the values of W—0. The plus sign indicates a greater amount of 

 vapor in the air of the forest; a minus sign the reverse. The units are 

 hundredths of a millimeter of mercury as it would be shown in a 

 barometer. The first part of the table is derived from the German 

 observations. To it are added the values calculated from the observa- 

 tions in forest and glade ( W—G), and in forest and plain ( IF— P), from 

 the Swedish service. 



At base of tree: 



G eneial average 



Deciduous 



E vergreens 



Young 



At tree crown : 



General average 



Deciduous 



Evergreens 



Swedish observations at base of tree 



W-G 



TT-P 



Tear. 



+0 

 4-2 

 

 +37 



-1-3 

 -2 



-f-4 



+ 10 

 -1-9 



These numbers are very small, so small that, notwithstanding the 

 absolute humidity is itself of small amount, they are usually less than 

 1 per cent of it. Tliis is true of the German observations, except for 

 the young trees. Here the value of W — is always positive and 

 reaches up to 4 or 5 per cent of the total absolute humidity. This 



