Page:Popular Science Monthly Volume 69.djvu/472

468 in the decade 1880-90, followed one of Brückner's wet periods, and the collapse of the 'boom' came when the drier period advanced. Farmers who went out on to the high plains in the years of slightly greater rainfall preceding the boom, and who lost all their capital, and more too, in the vain attempt to raise their grain in the years which followed, could with difficulty be convinced that the climate of the plains had not permanently changed for the worse. The impression left upon their minds, and upon the mind of anyone who saw the country later, was one of decreasing rainfall, unsuccessful agriculture and financial ruin. Within more recent years, in this same region of Kansas, with a somewhat increased rainfall during a wetter cycle, but without any permanent change to a wetter climate, the intelligent choice of cereals better adapted to the soil and climate, and the rational use of the available water supply, have wrought a wonderful change in the aspect and economic value of the state.

The following table shows the dates and characters of Brückner's periods:

Interesting confirmation of Brückner's thirty-five-year period has been found by Richter in the variations of the Swiss glaciers, but as these glaciers differ in length, they do not all advance and retreat at the same time. The advance is seen during the cold and damp periods. Supan has pointed out that the Brückner periods appear to hold good in the south polar regions. And Hann's study of the monthly and annual means of rainfall at Padua (1725-1900), Klagenfurt (1813-1900) and Milan (1764-1900) brings to light an alternation of wet and dry periods in harmony with the thirty-five year cycle. It should be noted that Brückner has found certain districts in which the phases and epochs of the climatic cycle are exactly reversed. These exceptional districts are almost altogether limited to marine climates. There is thus a sort of compensation between oceans and continents. The rainier periods on the continents are accompanied by relatively low pressures, while the pressures are high and the period dry over the oceans, and vice versa. The cold and rainy periods are also marked by a decrease in all pressure differences. It is obvious that changes in the general distribution of atmospheric pressures over extended areas, of the great centers of high and low pressure, are closely associated with fluctuations in temperature and rainfall. An oscillation of a few hundred miles one way or another may mean the difference between drought or plentiful rainfall over extended areas. These changes in pressure distribution must in some way be associated