Page:Popular Science Monthly Volume 82.djvu/318

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314
THE POPULAR SCIENCE MONTHLY

based our own belief on any such evidence as the foregoing, we ought to remember that our own sensation of heat, or cold, or dampness, by no means necessarily, or even usually, corresponds with the actual meteorological facts. Further, the great rainy and dry belts of the earth's surface are controlled by a world-wide distribution of temperature, pressure and winds, that is, by the general circulation of the atmosphere, and by conditions of the higher strata far and away beyond the reach of any local effects such as those of a forest. Universally, in response to natural controls, a scanty rainfall is hostile to tree-growth, and forests are favored by heavy rainfall, which gives good conditions of soil-moisture and is generally accompanied by higher relative humidity, more cloudiness and less extreme temperatures than prevail over treeless regions. In the case of mountains, again, it should be clearly in our minds that, as a rule, and up to a certain limit, an increase of altitude involves an increase of precipitation, quite apart from the presence or absence of any forest. We must be careful not to put the cart before the horse. The forests, in other words, are the result of the rainfall, and not vice versa.

 

Importance of the Subject: its Complexity

That this subject has an important relation to our national conservation policy no one will deny. Unfortunately, the discussion of it has become more or less a matter of semi-political controversy. Much has been written without adequate study of the question. Heated arguments, pro and con, have been advanced in debates and in print. Remarkably divergent views have been, and are to-day, held upon the question. It has been claimed that forests have no climatic influences whatever. On the other hand, some have believed that deforestation in North America has affected the climate of Europe. A recent writer maintains that the principal cause of the "intellectual and industrial stagnation" of the Spanish peasants is to be found in the effects of deforestation in making the climate drier, so that the people are "worked to death to support life." The literature is extended and bewildering. It runs back at least five hundred years. A bibliography published in 1872 contains nearly two hundred titles, and began with Fernando Columbus, who attributed the heavy rainfall of Jamaica to its heavy forests, and a (supposed) decrease of rainfall on the Azores and Canaries to deforestation. It has been said that this whole discussion first came up in really acute form at the time of the French Revolution, when private timberlands were largely destroyed.

The subject is thus greatly complicated by the nature of the discussion. It is, furthermore, by its very nature a complicated problem. On the one hand, climate itself is the complex resultant of many different controls. Among these are the latitude; the elevation above sea-level; the varying influences of land and water; the proximity of ocean currents; the prevailing winds and storms. In this list of controls, but at