Page:Popular Science Monthly Volume 40.djvu/835

Rh, which became more and more pronounced toward the interior regions. The years 1815, 1850, and 1881 came about the middle of relatively wet periods, and 1830 and 1860 of dry periods. The mean period of the oscillations was deduced from records of vintages, going back to the year 1400, to be thirty-six years. The changes appeared to be dependent on certain relations of atmospheric pressure, the wet periods being characterized by lesser differences, and the dry periods by increased differences, in that factor. The theory of a period of thirty-five or thirty-six years is fully elaborated by Prof. E. Brückner, of the University of Basle, in his book Klimaschwankungen (Vienna and Olmutz, 1890). Approaching the question from nearly every conceivable point of view—of temperature, precipitation, atmospheric pressure, the rise and fall and freezing and thawing of rivers, vintages and harvests—he is led to the same conclusion in every case. The period is nearly equivalent to three of the supposed eleven year sun-spot periods. Herr G. Hellman has counted thirty-four seasons since 1755 when December and January in Berlin were warmer than the average; but the warm seasons came at irregular intervals, and did not suggest any law.

Dr. W. Koppen, of Hamburg, records, as the outcome of an investigation which he made of the periodicity of weather changes, "that for certain intervals strongly marked periodical influences make their appearance and then vanish entirely, at times being replaced by others of a totally different character. No law has, as yet, been discovered for these changes."

The presence of forests has not been shown to contribute directly to the increase of rainfall, nor their removal to diminish it. Yet their influence on climate must be considerable. This is confessed when the farmer on the prairies plants belts of trees between his fields and the quarters from which cold winds and destructive storms are expected. They stand like a wall to protect the localities they overhang against sudden extremes of temperature and other accidents of violent weather. Although they may not increase the amount of precipitation to a perceptible extent, they, by means of their matted roots and the undergrowth which they promote, and by their beneficent shade, convert the ground on which they stand into a kind of reservoir, and husband the moisture which, without them, would run off or dry up at once. Thus they contribute to prevent sudden floods in the wet season, and, permitting a slow exudation of moisture into the streams, to keep them lively and the rivers to which they are tributary full during dry seasons. Many persons believe, too, that they diffuse a coolness and vaporous moisture in the atmosphere, the presence and influence of which, although they are not manifested in rain, are nevertheless real. Whether they may