336 PAST CLIMATES AND CLIMAXES. 



in terms of competition, ecesis, and reaction, as well as in the quantity of green 

 or dry matter produced (c/. Chapter XV). 



Douglass's 21-year cycle. — ^Douglass (1914* 119) has foimd that the curve 

 of tree-growth in northern Arizona shows a probable 21-year cycle during the 

 past 500 years. Lockyer has found a marked 19-year cycle in barometric 

 pressures in Australia and South America, and Douglass seems to assume that 

 this is probably the same as the 21-year cycle. The crests and means of the 

 latter are said to follow each other with great regularity for more than 400 of 

 the 500 years concerned. This cycle is weU-marked from 1410 to 1520 and 

 from 1610 to the present, though it shows several glaring discrepancies from 

 1520 to 1610. We have at present no further knowledge of this cycle, apart 

 from Huntington's suggestion that it may be responsible for the faint strands 

 found by Free about Owens Lake in California. 



Bruckner's 35-year cycle. — The existence of a 35-year cycle was first ad- 

 vanced by Briickner (1890), as a result of his studies of the periodic changes 

 in the water-level of the Caspian Sea. His conclusions were reinforced by 

 the investigations of Rykatchew, who found corresponding periods in the 

 temperature and rainfall of the region. The study of other inclosed basins 

 indicated that the rainfall of the entire globe showed periodic variations 

 essentially identical with those of the Caspian region. The times of extremes 

 in the water-level were: minima 1720, 1760, 1798, 1835, 1865; maxima 1740, 

 1777, 1820, 1850, 1880. The general dry periods of the earth were 1831 to 

 1840 and 1861 to 1865; the wet periods 1846 to 1855 and 1876 to 1880. 

 Regions were also found in which the direction of change was reversed; these 

 belonged almost wholly to oceanic regions. From a study of barometric 

 pressiu-es over continents and oceans, Bruckner reached the conclusion that 

 each rainy phase is accompanied by a reduction in the differences in baro- 

 metric pressure and each dry phase by a rise in the differences. Dry phases in 

 Eiu-asia were marked by a lowering of the barometric minimvun over the north 

 Atlantic, a heightening of the ridge of high pressure which extends from the 

 Azores northeast across central Europe to Russia, a deepening of the trough 

 of low pressure over the northern part of the Indian Ocean and the Chinese 

 Sea, a reduction of the barometric maximimi over Siberia, and through a 

 general increase of the amphtude of the annual variation. In the interior of 

 the continents, the fluctuations of rainfall appear with greater amplitude than 

 along the coasts. In western Siberia more than twice as much rain may fall 

 during the wet as during the dry phases, while the general amplitude is only 

 12 per cent. The mean temperature shows a similar periodicity. 



Bruckner expressed no opinion as to the causes of the 35-year period, but 

 the recently determined sun-spot cycle of 33 to 35 years appears to constitute 

 the explanation. Moreover, the phenomena just described strongly suggest 

 that this larger sun-spot cycle is translated into climate through the medium 

 of cyclonic storms, as already indicated for the 11-year cycle. Hansky has 

 found a larger period of about 72 years for the interval between the absolute 

 maximum and TninirmiTn of sun-spots, and Lockyer has noted a period of 35 

 years, which is indicated by the magnetic epochs as well as by sun-spot vari- 

 ations. Douglass (1914^:121; 1914:331) found an approximate 33-year 

 cycle in the growth of trees during the last 200 years. The exact period of 

 33.8 years is said to fit very well since 1730, and very poorly before that, 

 though without entire disagreement. Huntington places the 35-year cycle 



