178 



ANNUAL BEPORT SMITHSONIAN INSTITUTION, 193 3 



The separation between the first and second of these occurrences 

 is almost exactly 23 years, but there is a delay of nearly a year in the 

 appearance of the third. A similar delay marks, however, all of the 

 features from 1899 to 1918, after which the cycle returns approxi- 

 mately to its earlier phase-status. Compare, for illustration, the 

 year 1929 with 1860. 



The 23-year cycle is recognizable in tree rings and in clay deposits 

 of the glacial period as indicated by figures 18 and 19. 



1922 1923 1924 1925 1926 1927 I92S 1929 1930 1931 1932 1933 



north platte, neb. 



departures from normal temperature 



(5-month running means) 



=: observed = predicted 



Figure 20. 



In some cases the 23-year cycle has features of high or low values 

 prevailing over the course of several years and repeated similarly 

 during each cycle. Such cases occur, for example, in the Nagpur 

 precipitation cycles from the twenty-first through to the fifth year, 

 during which 7 years the precipitation is subnormal. This indi- 

 cates a probability that the subnormal precipitation will be experi- 

 enced in central India from 1942 to 1948. 



When the attempt is made to forecast weather for coming years 

 in more detail than such general statements as these, the embar- 

 rassing changes of phase already referred to are encountered. 

 These, though they do not destroy the general sequence of the indi- 

 vidual features of the 23-year cycle, produce displacements, some- 

 times as great as a year, and often several months, in the times of 

 their occurrence. Further research, it may be hoped, will aid in 

 overcoming this difficulty. 



