104 



FOREST INFLUENCES. 



but liis results, as pnblislicd in tlie ComptcH Rciidns^ are frngmeiitary. 

 They are given in tlie accompanying- table as tliey were i^nblislied. 

 Tlie results in absolnte humidity were not published. Tlie observa- 

 tions over trees are comj)ared with those at the same height over fields 

 and about a thousand feet distant. The psychrometer was several feet 

 above the trees in the case of the deciduous trees, but close to the top 

 of the evergreens. 



Jamiai'y -- 

 r»^bniaVy - 

 Mixicli .... 



April 



May 



June 



.July 



Aii;;ii.st 



S(])ti'iiil)tr 

 October.-. 

 November 

 December. 



The surplus of relalivc humidity over deciduous trees is not great, 

 but that just over tlie foliage of evergreens is so large as to be signifi- 

 cant. The pubhshed results do not admit of a ready comi)utation of 

 the absolute humidity, but the surplus of the relative is great enough 

 to snggest a surplus of the absolute, large enongh to be easily meas- 

 ured. The special, elaborate, and refined observations at Ried, in 

 Austria, show slightly greater variations in absolute humidity than 

 previous ones, and though they are nearly evanescent, some uniformity 

 can be traced in them. As they were taken at irregular intervals they 

 are not suitaljlc for a general tabulation ami their number forbids their 

 being quoted at length. The conclusions reached by the discussion of 

 them and given by Dr. Lorenz Liburnau are as follows: 



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 interihe- 

 diate lietween the other two. This rate is least in the late hours of tlie 

 night, when it may be zero. It increases with the increase of the tem- 

 perature of the air, becoming greatest in the midday hours, when, 

 under exceptionally favorable circumstances, it may make a difference 

 of ]() per cent, or even more. Occasionally, in high winds, the absolute 

 humidity is greater over the trees. Over the field station the daily 

 IH'ogress of the absolute humidity was about the same as in the forest, 

 but the maximnm difference was only about half as great. The abso- 

 lute humidity in and al)ove the forest is greater than that over the open 

 fields, and there is some trace of an increase of this difference to the 

 time of maximum. 



