. J5 ._, DR. C. CHREE: SOME PHENOMENA OF SUNSPOTS 



Eventually a practical and economical plan suggested itself. Before adopting it I 

 had assured myself that the 27-day phenomenon applied to quiet days. It then 

 became clear that if one selected 5 quiet days for each month, and considered the 

 days which followed them after any given interval, as well as the days following the 

 selected disturbed days after the same interval, it was necessary to consider only a 

 comparatively few consecutive days near the date when the pulse was expected to 

 appear. For instance, days from 79 to 84 days subsequent to the 5 selected 

 disturbed days of January, 1906, are practically contemporaneous with days from 79 

 to 84 days subsequent to the 5 selected quiet days of the same month. If there is an 

 appreciable pulse with crest (or hollow) about 81 days subsequent to the represen- 

 tative disturbed or quiet days, this will be rendered manifest by the differences 

 between the two sets of subsequent days, irrespective of what the appropriate 

 average character figure from all days might be. 



By this time I had also discovered that the 27-day period is as clearly recognisable 

 in days which precede as in those which follow selected disturbed days. It was thus 

 decided to consider days before as well as days after the selected days, and to go 

 equally far in both directions. It was also decided to take the later period, 1906 to 

 1911, so as to have an international basis for the selected days, whether quiet or 

 disturbed. The quiet days were those actually selected at de Bilt. 



The final mean results of the investigation are given in Table III., p. 254, and are 

 shown graphically in fig. 2. But for considerations of time, it would have been desirable 

 to take more than six days near the epochs where the pulses were expected. 



The columns headed D and Q respectively in Table III., refer to the days associated 

 with the selected disturbed days and to those associated with the selected quiet days. 

 The number of selected days used was always the same for the disturbed and the 

 quiet days, but varied, as shown in the second line, because only parts of the first 

 and last years of the series could be utilised. For example, when dealing with the 

 days which were from 84 to 79 days prior to selected days, April 1906 was the 

 earliest month whose selected days one could employ. For that particular quest the 

 15 selected days of the first 3 months of 1906 had to be omitted, leaving only 345 

 selected days. Similarly, as no data subsequent to December 1911 were to be used, 

 the last 15 selected days of 1911 had to be omitted when dealing with the days 

 79 to 84 days subsequent to selected days. January 1, 1906, was a selected quiet 

 day, and December 31, 1911, a selected disturbed day. Thus the earliest and the 

 latest of the selected days, both quiet and disturbed, were omitted from the central 

 group of days n-3 to n+3, leaving 358 available. 



The "character" figures in the third line of Table III. relate to the periods 



covered by the corresponding selected days. Thus 0'659 given for the group of days 



84 to n-79 is the mean for the period commencing April 1, 1906, and ending 



In some ways it would have been better to have replaced this 



by a mean applicable to the period containing the days which preceded the selected 



