76 



CLIMATIC CYCLES AND TREE GROWTH 



surface. The ultra-violet rays measured by Pettit are short waves and have 

 little effect in heating. They are thought to produce changes in ozone and 

 ionization of our upper atmosphere and in plant growth, with other ef- 

 fects upon climate and plants not yet worked out. The proportionate 

 changes from day to day and year to year are far greater in the ultra-violet 

 than in the general heat radiation. And the source of the ultra-violet is 

 doubtless different. The short series of observed data, 1924 to 1931 or later, 

 shows in parts better resemblance between ultra-violet radiation and sunspot 

 numbers than is found between total heat radiation and sunspot numbers. 



1 



1924- 



1925 



1926 



1927 



1928 



1929 



1930 



1931 



53 »- 4 



1.2 



JP 



s 



1.0 



100 



50 



Fig. 32 — Ultra-violet radiation compared to monthly sunspot numbers 

 and the solar constant. 



This intimates a substantial difference in source and points more directly to 

 the activity in the spots themselves. Pettit has called attention to an interval 

 of possibly reversed correlation from the middle of 1928 to early in 1930. 

 Pettit's curve and the monthly sunspot numbers are compared in figure 32. 



INTERNATIONAL MAGNETIC CHARACTER FIGURE C 



The International Magnetic Character Figure C, represents the daily 

 "agitation" or rapid irregular changes in the intensity of terrestrial magne- 



