Page:Popular Science Monthly Volume 69.djvu/473

Rh with changes in the general circulation of the atmosphere, and these again must depend upon some external controlling cause or ca W. J. S. Lockyer has called attention to the fact that there seems to be a periodicity of about thirty-five years in solar activity, and that this corresponds with the Brückner period. This longer cycle, underlying the sunspot period, alters the time of occurrence of the sunspot maxima in relation to the preceding sunspot minima. He makes out three periods in solar activity, of between three and four years, about eleven, and about thirty-five year respectively. These are related as 1:3:9.

It is clear that the existence of a thirty-five-year period will account for many of the views that have been advanced in favor of a progressive change of climate. A succession of a few years wetter or drier than the normal is likely to lead to the conclusion that the change is permanent. Accurate observations extending over as many years as possible, and discussed without prejudice, are necessary before any conclusions are drawn. Observations for one station during the wetter part of a cycle should not be compared with observations for another station during the drier part of the same, or of another cycle.

Climatic Cycles of Longer Period.—There are evidences of longer climatic cycles than eleven or thirty-five years. Brückner calls attention to the fact that sometimes two of his period; seem to merge into one. Richter shows much the same thing for the Alpine glaciers. James Geikie, in Scotland, has brought forward evidence of several climatic changes in postglacial times. Blytt, in Norway and Sweden, finds some botanical evidence of four great climatic waves since the last glacial period. Brögger estimates that a mean annual temperature between 3° and 4° higher than the present was found in the Christiania Fjord in postglacial time. Lorié, in Holland, finds confirmation of Blytt's views. Gradmann, on botanical evidence, beliein a warmer climate in central Europe after the last ice age, and then a cooler one. dough concludes that a three hundred-years cycle exists in solar and terrestrial phenomena, the thirty-six year cycle being as it were superimposed upon the longer one. Kingsmill reports a periodicity of three hundred years in droughts and famines in northern China. And so on. As yet. nothing sufficiently definite to warrant discussion here ha; been brought forward.

Geological Changes in Climate.—Changes of climate in the geological past are known with absolute certainty to have taken place: periods of glacial invasions, as well as periods of more genial conditions. The evidence, and the causes of these changes have been discussed and re-discussed, by writers almost without number, and from all points of view. Changes in the intensity of insolation; in the sun itself; in the conditions of the earth's atmosphere; in the