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And what is the influence of the forest on summer temperature 1 In the first place 

 the body-heat of trees is far lower than the summer temperature. Then, since trees in 

 summer are constantly elevating enormous quantities of water from the earth to the air, 

 they cool the air by so much as the earth's moisture is cool. Trees not only elevate water, 

 but evaporate it into the air, and, knowing this, it is no extravagance to say that every 

 leaf is a refrigerator the whole summer long. Turning our thoughts again to the forest 

 soil, we find that, sheltered from the sun, it slowly gives up by evaporation the moisture 

 accumulated through the winter, and in that way greatly cools the atmosphere. Every 

 process whereby the air is warmed in the forest in winter, is reversed or discontinued in 

 the summer, and every breath that comes from the woods in summer is tempered by some- 

 thing of the winter's cold. 



In 1875 Ebermeyer of Bavaria enlightened us as to this power of the forest to 

 equalize the diurnal and seasonal vibrations of temperature, and his doctrines have been 

 confirmed by further tests. He found that during the day there was, in summer, a steady 

 outflow of cool air from the forest at the level of the earth, while during the night the 

 forest remained warmer, and received an inflow of air from the open country. Extending 

 his thermometric studies over many months he found that the maximum temperature of 

 the forest was 9° F. lower than the maximum in adjacent open spaces, and the minimum 

 temperature of the forest was nearly as much higher than in the adjacent open spaces. 



The forest, again, has a governing, restraining, conservative influence on another of 

 the conditions which constitute a climate, namely, the humidity of the atmosphere. The 

 secret of this influence lies partly in the soil. The forest soil, in the first place, is tra- 

 versed by roots, living and dead, and these are so many conduits to conduct rain-water 

 into the soil. Over the subsoil so pierced and traversed by roots, is a heavy layer of 

 humus, highly absorbent and hygroscopic, over all this is a layer of leaves, twigs, branches, 

 bark and moss, and, as our great, dead master has taught us in his latest lesson, through 

 all and above all these layers of the forest soil are the channels of earth worms, and the 

 fine earth which they have elaborated. So constructed, the soil of woods is actually an 

 enormous sponge, with incredible power of arresting and detaining rain-water ; and, sat- 

 urated in cool, rainy seasons it becomes a vast reservoir from which the parched air of 

 dry seasons acquires needed humidity. Nor must we in this connection forget to mention 

 again the enormous evaporation from the leaves — which, by the way, is greatest when it 

 is most needed. 



How just a treasurer is the forest, then, of all the elements of a good climate ! It is 

 an agent that can mitigate the fierce heats of summer, and detain and warm the biting 

 winds of winter ; and hoard up the redundant rains to yield a portion again when sum- 

 mer has parched soil and air alike. Impressed by truths like these, Alexander Buchan 

 has declared that the climate of forests is so equable, that it is comparable only to the 

 climate of islands. 



But what has all this to do with health 1 A variable climate is an unwholesome 

 climate. Other things being equal — an equable climate is a good climate. Sudden ex' 

 cess of heat is enervating ; sudden excess of cold is the common breeder of catarrhal dis- 

 eases in general, of pulmonary diseases, acute and chroiiic, and of the more acute inflam- 

 matory diseases of the abdominal viscera. 



Of the relations of health to atmospheric moisture we know only this : that sudden 

 and incessant changes are injurious. There are salubrious dry climates, and salubrious 

 moist climates, but the climate that is notably dry to-day and notably moist to-morrow, is 

 not a good one from the physician's stand- point. 



A common objection to these doctrines is often put in this shape: — Climate is a re- 

 sult of world-wide causes. Only a small portion of the earth's surface is dry land, and 

 only a small fraction of the land can ever be covered with forest. The influence of the 

 forest upon climate is therefore trivial. 



The truth is, however, that while the general climate of the wide world is hardly af- 

 fected by the existence of forests, yet local climate is greatly modified, and, in almost all 

 cases, is modified for the better. The natural investment of animals, and our own artificial 

 clothing does nothing to modify the temperature of the world or of the cosmos, yet how in- 

 valuable is it to the individual ! 



