160 TREES AS SHELTER TO GROUND TO THE LEEWARD. 
quieseent, 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 obyious is its effect, 
and no one can have passed from the field to the wood in cold, 
windy weather, without having remarked it.* 
lowers the temperature of the air within it, so far it creates upward or down- 
ward currents in the atmosphere above it, and, consequently, a flow of air 
towards or from itself. These air-streams have a certain, though doubt- 
less a very small, influence on the force and direction of greater atmospheria 
movements. 
* 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 sensible 
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 insupportable. 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 
to the immobility of the air, is, that sounds are transmitted to incredible 
distances in the unbroken forest. Many instances of this have fallen under 
my own observation, and others, yet more striking, have been related to 
me by credible and competent witnesses familiar with a more primitive 
condition of the Anglo-American world. An acute observer of natural 
phenomena, whose childhood and youth were spent in the interior of one 
of the newer New England States, has often told me that when he estab. 
lished his home in the forest, he always distinctly heard, in still weather, 
the plash of horses’ feet, when they forded a small brook nearly seyen- 
eighths of a mile from his house, though a portion of the wood that inter- 
vened consisted of a ridge seventy or eighty feet higher than either the house 
or the ford. 
Ihave no doubt that, in such cases, the stillness of the air is the most 
important element in the extraordinary transmissibility of sound; but it 
must be 2dmitted that the absence of the multiplied and confused noises, 
