294 



Dr. W. J. S. Lockyer. 



accurate, made it unnecessary to discuss the variation (as shown in the 

 case of the sunspot areas) regarding the total areas of the curves from 

 minimum to minimum. This variation seems to be more pronounced 

 in the curve representing the horizontal force than in that representing 

 declination. 



Length of the Period of Variation thus determined. 



In summing up the values obtained for the length of the secular 

 period of variation under discussion, we form the following table : — 





Maximum to 

 maximum. 

 Years. 



Minimum to 

 minimum . 

 Years. 



Combined mean .... 



35-5 

 35-25 

 35 5 



33-3 



35-41 



33-3 



34-89 



The observations thus lead to the conclusion that underlying the ordi- 

 nary sunspot period of about eleven years there is another cycle of greater 

 length, namely, about thirty-five years. 



This cycle not only alters the time of occurrence of the maxima in relation 

 to the preceding minima, bid causes changes in the toted spotted area of the 

 sun from one eleven-year period to another. 



The Variation in the Length of the Interval Minimum to Minimum, 



Having found a definite variation in the length of the interval mini- 

 mum to maximum, the curves show a further variation when the 

 interval — minimum to minimum — was considered. An attempt was 

 therefore made to see if any law could be traced, but the inquiry only 

 led to a negative result. 



The following table contains the values for the periods — minimum to 

 minimum — and the differences from the mean, for both the sunspot and 

 magnetic curves individually and combined. It will be seen that the 

 alternation of signs in the columns showing the sunspot differences is 

 not corroborated by the magnetic differences, but when the combined 

 values are used this oscillation for consecutive periods is still en 

 evidence : — 



