164 
INFLUENCE OF FOREST ON HUMIDITY. 
the sun’s rays to the earth, and, of course, an elevation of tem¬ 
perature which would occasion a great increase of evapora¬ 
tion. As a mechanical obstruction, it impedes the passage of 
air currents over the ground, which, as is well known, is one 
of the most efficient agents in promoting evaporation and the 
refrigeration resulting from it.* In the forest, the air is almost 
quiescent, and moves only as local changes of temperature 
affect the specific gravity of its particles. Hence there is often 
a dead calm in the woods when a furious blast is raging in the 
open country at a few yards’ distance. The denser the forest 
—as for example, where it consists of spike-leaved trees, or is 
thickly intermixed with them—the more obvious is its effect, 
and no one can have passed from the field to the wood in cold, 
windy weather, without having remarked it.f 
tact in the higher regions, and discharge large raindrops, which, if not 
divided by some obstruction, will reach the ground, though passing all 
the time through strata which would vaporize them if they were in a state 
of more minute division. 
* It is perhaps too much to say that the influence of trees upon the 
wind is strictly limited to the mechanical resistance of their trunks, 
branches, and foliage. So far as the forest, by dead or by living action, 
raises or lowers the temperature of the air within it, so far it creates 
upward or downward currents in the atmosphere above it, and, conse¬ 
quently, a flow of air toward or from itself. These air streams have a 
certain, though doubtless a very small influence on the force and direction 
of greater atmospheric movements. 
t As a familiar illustration of the influence of the forest in checking 
the movement of winds, I may mention the well-known fact, that the sen¬ 
sible cold is never extreme in thick woods, where the motion of the air is 
little felt. The lumbermen in Canada and the Northern United States 
labor in the woods, without inconvenience, when the mercury stands 
many degrees below the zero of Fahrenheit, while in the open grounds, 
with only a moderate breeze, the same temperature is almost insupport¬ 
able. The engineers and firemen of locomotives, employed on railways 
running through forests of any considerable extent, observe that, in very 
cold weather, it is much easier to keep up the steam while the engine is 
passing through the woods than in the open ground. As soon as the train 
emerges from the shelter of the trees the steam gauge falls, and the stoker 
is obliged to throw in a liberal supply of fuel to bring it up again. 
Another less frequently noticed fact, due, no doubt, in a great measure 
