in:, c. CHI;I:I: s..\n: nil NO.MHNA OF SUNSPOTS 



(1 effort to direct attention to the difficulty which seems to be raised by tin- 

 oonspininiis nature of the 27-day period in quiet days. The rapidity of the decline 

 in disturlKim-e and the rapidity of its resuscitation after the representative quiet day 

 are prominent facts. It will hardly, I think, be suggested that there are limited solar 

 I -similar to sunspots in dimensions whose direct presentation to tin- earth 

 exerte a soothing or damping influence on magnetic disturbance on the earth, removing 

 or diminishing disturbances which otherwise would have made their presence felt. 



22. A serious difficulty in the way of an exact determination of the period is that 

 magnetic storms, and magnetically quiet times, are events usually covering a large 

 iiumW of hours. A magnetic storm is seldom confined to a single day. Successive 

 magnetic strms do not as a rule present closely similar features, nor are they usually 

 of closely similar length. There is thus as a rule no such thing as a definite interval 

 between them. In the majority of cases opinions would differ often by hours as to 

 when a magnetic storm logins, and still more so as to when it ends. The uncertainty 

 is least about the time of commencement, and that is presumably the reason why 

 Mr. MAUNDER calculated his intervals from the times of commencement. If, however, 

 one could assign exact intervals for the beginning and ending, the natural interval 

 would seem to be, not the time from beginning to beginning, but the time from centre 

 to centre. If wo accept a jet theory, then if the second of two magnetic storms is 

 shorter than the first, the jet and so presumably the corresponding solar area has 

 contracted. In the absence of definite knowledge to the contrary, the most natural 

 hypothesis would seem to be that the jet has contracted uniformly about its centre. 



If successive magnetic storms were of roughly equal duration, and if in a number of 

 instances they both had what are termed " sudden commencements," much less 

 uncertainty would attach to the interval. As I pointed out, however, in a review of 

 Mr. MAUNDEK'S first paper, Nature but seldom presents this simple case. Of the 

 276 magnetic storms which Mr. MAUNDER'S list gave for Greenwich between 1882 

 and 1903, only 77 had "sudden commencements." Of the 91 storms which he 

 regarded as showing a 27-28 day period during these 22 years, only 15 had "sudden 

 commencements," and there were only four cases in which two successive storms of 

 his sequence groups had both " sudden commencements." 



The definition of a magnetic storm is purely arbitrary. A striking example of this 

 is afforded by the Kew and Greenwich lists of " character " figures supplied in 190G, 

 1907 and 1908 to de Bilt. In both lists the days of character " 2" i.e., magnetic 

 storms according to Greenwich standard numbered 29, but only 19 of these days 

 were common to the two lists. Both lists gave eleven 2's in 1907 ; but the 

 Greenwich list gave nine 2's in each of the years 1906 and 1908, while the Kew 

 list gave five in the former year and thirteen in the latter. Thus if attention is 

 confined to "magnetic storms," where one man gets a sequence of approximately 

 the right period, another gets no sequence at all. If, on the other hand, one takes 

 disturbances moderate as well as large, the number is so great that it does not require 



