192 CONFLICTING INFLUENCES. 
tends, on the one hand, to diminish the humidity of the air by 
sometimes absorbing moisture from it, and, on the other, to in- 
crease that humidity by pouring out into the atmosphere, in a 
vaporous form, the water it draws up through its roots This 
last operation, at the same time, lowers the temperature of the 
air in contact with or proximity to the wood, by the same law 
as in other cases of the conversion of water into vapor. 
As I have repeatedly said, we cannot measure the value of 
any one of these elements of climatic disturbance, raising or 
lowering of temperature, increase or diminution of humidity, 
nor can we say that in any one season, any one year, or any one 
fixed cycle, however long or short, they balance and compen- 
sate each other. They are sometimes, but certainly not always, 
contemporaneous in their action, whether their tendency is in 
the same or in opposite directions, and, therefore, their influ- 
ence is sometimes cumulative, sometimes conflicting ; but, wpon 
the whole, their general effect is to mitigate extremes of atmos- 
pheric heat and cold, moisture and drought. They serve as 
equalizers of temperature and humidity, and it is highly prob- 
able that, in analogy with most other works and workings of 
nature, they, at certain or uncertain periods, restore the equi- 
librium which, whether as lifeless masses or as living organ- 
isms, they may have temporarily disturbed.* 
* There is one fact which I have nowhere seen noticed, but which seems to 
me to have an important bearing on the question whether forests tend to 
maintain an equilibrium between the various causes of hygroscopic action, 
and consequently to keep the air within their precincts in an approximately 
constant condition, so far as this meteorological element is concerned. I 
refer to the absence of fog or visible vapor in thick woods in full leaf, even 
when the air of the neighboring open grounds is so heavily charged with con- 
densed vapor as completely to obscure the sun. The temperature of the at- 
mosphere in the forest is not subject to so sudden and extreme variations as 
that of cleared ground, but at the same time it is far from constant, and so 
large a supply of vapor as is poured out by the foliage of the trees could not 
fail to be sometimes condensed into fog by the same causes as in the case of 
the adjacent meadows, which are often covered with a dense mist while the 
forest-air remains clear, were there not some potent counteracting influence 
always in action. This influence. I believe, is to be found partly in the 
equalization of the temperature of the forest, and partly in the balance be- 
