VARIATIONS IN CLIMATE. 813 



tions, wliich became more and more pronounced toward the in- 

 terior regions. The years 1815, 1850, and 1881 came about the 

 middle of relatively wet periods, and 1830 and 18G0 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 atmos- 

 pheric pressure, the wet jieriods being characterized by lesser dif- 

 ferences, 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. Bruckner, 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 press- 

 ure, 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 irregu- 

 lar 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 di- 

 rectly 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 pro- 

 tect the localities they overhang against sudden extremes of tem- 

 perature 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 under- 

 growth which they promote, and by their beneficent shade, con- 

 vert 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 atmos- 

 phere, the presence and influence of which, although they are 

 not manifested in rain, are nevertheless real. Whether they may 



