CHANGES OF CLIMATE 469 



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 Bruckner period. This longer cycle, under- 

 lying 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- ctively. 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 per- 

 manent. Accurate observations extending over as many years as 

 possible, and discussed without prejudice, are necessary before any con- 

 clusions 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 Peri/jd. — There are evidences of longer 

 climatic cycles than eleven or thirty-five years. Bruckner calls atten- 

 tion to the fact that sometimes two of his period; seern to merge into 

 one. Eichter 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. Brogger estimates that a mean annual tempera- 

 ture between 3° and 4° higher than the present was found in the 

 Christiania Fjord in postglacial time. Lorie, in Holland, finds con- 

 firmation of Blytts views. Gradmann, on botanical evidence, belie - 

 in 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. Kingsrnill reports a period- 

 icity 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 geo- 

 logical past are known with absolute certainty to have taken place : 

 periods of glacial invasions, as well as periods of more genial condi- 

 tions. The evidence, and the causes of these changes have been dis- 

 cussed 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 astrono- 



