286 



Dr. W. J. S. Lockyer. 



his own words, that " la frequence cles taches solaires persiste a changer 

 periodiquement depuis leur decouverte en 1610; que la longueur 

 moyenne de la periode est de 11J ans, et que cette meme periode satis- 

 fait aux changements de la variation magnetique, et meme de la 

 frequence des aurores boreales." 



Dr. Wolf was careful to point out that it was only the mean length 

 of the solar period that covered a period of 11- years, and that the real 

 length of any one period might differ from this value by as much as 

 two years. The form in which he stated this result* was 



T = 11-1U ± 2,030 (als Schwankung) ± 0,307 (als Unsicherheit) ; 



where T represented the length of the period, ± 2,030 the variation 

 from the mean value, and ± 0,307 the probable error of the deter- 

 mination. 



His attention was also drawn to the fact that the times of maxima 

 did not occur a constant number of years after a preceding minimum, 

 and he was led to determine the mean time of occurrence of the maxi- 

 mum after the preceding minimum and of the minimum after the 

 preceding maximum, giving the mean intervals as 4*5 and 6 '5 years 

 respectively. 



Further, he at first concluded that the total spotted area for each 

 period was nearly constant, but, as he later remarks,! this view could 

 not be held, as these quantities not only varied but indicated " eine 

 bestimmte Gesetz-massigkeit." The length of the period of this varia- 

 tion he gave as about 178 years, which covered practically sixteen 

 ordinary sunspot periods. ("11,1111 x 16 = 177,7777.") 



Somewhat later Dr. Wolf was led to suggest a shorter period of 

 55*5 years, which comprises about five ordinary eleven-year periods. 



In a recent paper J Professor Simon Newcomb has published the 

 results of his investigation of the irregularities in the successive sun- 

 spot periods, using as a basis Dr. Wolf's numbers up to the end of 

 1872, and the spot areas as derived from the Greenwich reduction of 

 the solar photographs taken daily at Greenwich, Dehra Dun, and 

 Mauritius. 



The final conclusion at which he arrives is summed up in the follow- 

 ing paragraph : — 



"Underlying the periodic variations of spot-activity there is a uni- 

 form cycle unchanging from time to time and determining the general 

 mean of the activity." 



Professor Newcomb mentions, however, no length of period for this 

 cycle, but speaking of its origin he remarks, " whether the cause of 

 this cycle is to be sought in something external to the Sun or within 



* ' Astron. Mittheil.,' Wolf, 187 ; p. 40. 

 t * Astron. Mittheil.,' 1876, p. 47 et seq. 

 X ' The Astro-Physical Journal,' vol. 13, No. 1, 1901, p. 1. 



