250 DR. C. CHREE: SOME PHENOMENA OF SUNSPOTS 



group thus representing a specially high grade of disturbance. There were in all 103 

 ,,f tli.-sr days, the annual number varying from 15 in 1907 to 20 in 1906. The 

 following were the mean character figures found for the primary day and the 

 subsequent days indicated : 



This gives a period if anything in excess of 28 days, and so suggests a slight 

 increase in the length of the 27-day period as the intensity of the primary disturbance 

 is increased ; but a considerably larger number of days, and so a considerably longer 

 period of years, would be required to establish the result. 



The mean "character" figures given above for days n + 28 to n + 30 are dis- 

 tinctly larger than the corresponding figures in Table I., but the excess in these 

 days is relatively less than that on day n itself. Thus the excess in the 

 "character" figure given above for day n + 28 over the average day of the 

 six years (i.e., 0'871 0'663 = 0'208) is only 20 per cent, of the excess on day 

 n(l'683 0'663 = 1'020), while the corresponding percentage from Table I. was 27. 



7. If individual magnetic storms are directly due to individual sunspots, as 

 various writers have suggested, it is, of course, a natural inference that when the 

 sun's rotation has brought a spot round to the position it occupied relative to the 

 earth when a magnetic storm occurred, a second storm will be experienced. This 

 seemingly is what led HARVEY and MAUNDER independently to suggest a 27^-day 

 period for magnetic storms. 



Our previous investigations show a period of about 27 days, which, however, is not 

 confined to what are usually termed " magnetic storms," but belongs equally to 

 moderate disturbances, which are frequent events. If, then, magnetic storms are due 

 to sunspots, equally so it would seem must be the minor disturbances ; and if 

 magnetic storms sometimes recur, as Mr. MAUNDER and the Rev. A. L. CORTIK 

 believe, at several reappearances of one and the same sunspot, the same thing is to 

 be expected of minor disturbances. This implies that " character " figures should 

 show a pulse near day n + 54, as well as near day n + 27. 



This conclusion, however, seems a natural one apart from all theory. The 

 impression left on my own mind after a study of the " character " figures was that a 

 tendency existed for the magnetic conditions, whether disturbed or not, to be in some 

 way related to or as biometricians would say correlated with the magnetic conditions 

 prevalent 27 days earlier or kter. Tbe days forming columns n + 26 to n + 30 in 

 Table I., or in the corresponding table for the years 1890 to 1900, are disturbed 

 sensibly more than the average day, and we should thus expect more than average 

 disturbance on days n + 53 to + 57, with a culmination about days n + 54 and n + 55, 



