Page:Popular Science Monthly Volume 2.djvu/209

 streams receive a continual supply, while its evaporation from the surface and its transpiration through the leaves of the trees afford moisture to the atmosphere.

The moisture thus imparted to the atmosphere mitigates the severity of a drought in various ways: First, by lessening evaporation from the surface, as this is accelerated by a dry atmosphere, and retarded by a moist one; secondly, by affording to the soil a greater proportion of moisture for condensation, when the surface cools at night. Thirdly, by affording moisture for direct absorption by the leaves. This is a disputed point among men of science; but the late researches of Cailletet promise a reconciliation of the conflicting views, as usual, by showing that both were wrong and both right. He found that, when a plant was abundantly supplied with moisture in the soil, the leaves never absorb; but that they do absorb whenever the soil is deficient in moisture, and the leaves begin to droop.

Hence, there might be evidence of drought in a country without forests, while there were no such evidences in a country sufficiently guarded by forests, though the amount of rain had been the same in both.

These considerations are not altogether without value in regard to fruit-growing. It is true that grapes do best in rather a dry climate; but most kinds of fruit require considerable moisture, especially at the time of transplanting, and also when the fruit is maturing. In affording some mitigation of the extreme effects of dry weather, forests may be regarded as having a beneficial influence on the growing of most fruits.

Of far greater concern to the fruit-interests of any locality is the influence of woodland on temperature. On this subject there is quite a general unanimity of opinion. Certain forms of the evidence are so familiar that the conviction produced is general. Every one, who has travelled through woodlands and open fields during cold weather, has readily perceived how much warmer was the atmosphere of the wood than that of the field. It is said that engineers on our railroads find that it requires less fuel to keep up steam in passing through a long stretch of woodland (Marsh). But the warming influence of the forest has been subjected to more rigid tests than these. Boussingault proved that, within several parallels of the equator on either side, the temperature of cleared land is about two degrees higher than that which is covered with forest. But we are more directly concerned with results in our temperate climate.

The researches of Becquerel, Krutsch, and Berger, had appeared to prove, first, that a wooded region would have a cooler summer and a warmer winter than a region almost destitute of woods; and, secondly, that during the daytime the temperature of the atmosphere in the forest would always be lower, and during the night always higher, than in the open field; the difference between the diurnal maximum and