INFLUENCE OF THE FOREST ON PRECIPITATION. 187 
Lovelace on Climate, etc.), and, on the other hand, rain has 
become more frequent in Egypt since the more vigorous culti¬ 
vation of the palm tree.” 
Hohenstein remarks : “ With respect to the temperature in 
the forest, I have already observed that, at certain times of 
the day and of the year, it is less than in the open field. 
Hence the woods may, in the daytime, in summer and to¬ 
ward the end of winter, tend to increase the fall of rain ; but it 
is otherwise in summer nights and at the beginning of winter, 
when there is a higher temperature in the forest, which is not 
favorable to that effect. * * * The wood is, further, like 
the mountain, a mechanical obstruction to the motion of rain 
clouds, and, as it checks them in their course, it gives them 
occasion to deposit their water. These considerations render 
it probable that the forest increases the quantity of rain; but 
they do not establish the certainty of this conclusion, because 
we have no positive numerical data to produce on the de¬ 
pression of temperature, and the humidity of the air in the 
woods.” * 
Barth presents the following view of the subject: u The 
ground in the forest, as well as the atmospheric stratum over 
it, continues humid after the woodless districts have lost their 
moisture ; and the air, charged with the humidity drawn from 
them, is usually carried away by the winds before it has de¬ 
posited itself in a condensed form on the earth. Trees con¬ 
stantly transpire through their leaves a great quantity of moist¬ 
ure, which they partly absorb again by the same organs, while 
the greatest part of their supply is pumped up through their 
widely ramifying roots from considerable depths in the ground. 
Thus a constant evaporation is produced, which keeps the 
forest atmosphere moist even in long droughts, when all other 
sources of humidity in the forest itself are dried up. * * * 
Little is required to compel the stratum of air resting upon a 
wood to give up its moisture, which thus, as rain, fog, or dew, 
is returned to the forest. * * * The warm, moist currents 
* Der Wald , p. 13. 
