SUNSPOTS— ANDREWS 141 



ologists that solar influences dominate the weather situation of the 

 earth, and the attempts being made at present to discover the clue 

 to long-range weather forecasting have their basis in the study of 

 the output of heat from the sun. 



The second correlation between a solar phenomenon and sunspot 

 variation is that of the ultraviolet content of the sun's radiation. At 

 times of sunspot maximum the percentage of ultraviolet radiation 

 from the sun reaches a maximum; at sunspot minimum, it is at a 

 minimum. In this correlation lies the clue to a large number of 

 terrestrial effects. The biological effects of ultraviolet light might 

 form the subject of a thesis by a biologist. In general, I believe, 

 the thesis would hold that ultraviolet light is conducive of good 

 health ; at least, such reasoning has resulted in the fad of sun bathing, 

 either for the acquisition of bodily tone or for the acquisition of a 

 fashionable bodily tan. One might suggest that it is most economi- 

 cal to acquire a coat of tan at times of sunspot maximum, for then 

 the ultraviolet content of sunlight, the motivating factor in the crea- 

 tion of tan, is at a maximum and the desired coat may be acquired in 

 the minimum interval of time. 



The influence of ultraviolet light upon plant life is that of stim- 

 ulating growth, though it must be admitted that frequent rains will 

 also assist the weeds to grow luxuriantly amid the prouder foliage 

 of your garden. If you will grant the earlier theory that rainfall 

 should be at a maximum at the time of sunspot maximum, then the 

 coupling of frequent rains with the maximum receipt of ultraviolet 

 radiation should manifest itself in plant life on the earth. A very 

 simple manner of testing the hypothesis is by the study of the growth 

 of trees. Even as little children we learn that a tree adds a ring 

 each year, so that, by sawing down the old pine tree and counting 

 rings backward from the bark, it is possible to tell its age. The 

 theory would indicate that at times of sunspot maximum the width 

 of the ring should be greater than at times of sunspot minimum 

 or at intermediate times. The correlation is so good that Prof. 

 A. E. Douglass, of the University of Arizona, has used it to deter- 

 mine the antiquity of the Indian ruins in the southwestern areas of 

 the United States, and for carrying back our records of sunspot 

 activity prior to the time of satisfactory man-made records of it. 

 Sample cross-sectional borings of the timbers used in the construc- 

 tion of these dwellings show patterns of ring widths that are re- 

 produced time and again. Comparisons with the cross-sections of 

 recently felled patriarchs of the neighboring wooded areas and cor- 

 relation with recorded observations of sunspot activity have enabled 

 Professor Douglass to carry back his studies of tree-ring cycles to 



