, J46 PR. C. CHREE: SOME PHENOMENA OF SUXSI'OTS 



The " character " figures in each column were then added up as if they were purely 

 arithmetical quantities, and an arithmetic mean was taken. This was regarded as a 

 measure of the disturbance existent on the representative day of the column. Thus 

 the means for columns n, n-1, and + l represented respectively the amount of 

 disturbance on the typical selected disturbed day, and on the days immediately 

 preceding and following it. These mean " character" figures showed in the clearest 

 way the existence of a period somewhat in excess of 27 days, but no shorter period 

 was disclosed. This implied that if any day were considerably more disturbed than 

 the average day of the month, then the day 27 days subsequent to it was likely to be 

 also more disturbed than usual 



The acceptance of the arithmetic mean of a number of " character" figures as itself 

 a measure of magnetic disturbance is open to criticism on several grounds. There is no 

 strict line of demarcation between the three classes of days. There are in reality an 

 infinite variety of grades intermediate between the extremely quiet day, which cannot 

 get less than " 0," and the extremely disturbed day which cannot get more than "2." 

 Some days to which " 2 " is allotted represent disturbances whose energy on any 

 conceivable view must be immensely more than twice possibly more than twenty 

 times the energy of disturbance on the average day of character " 1." The 

 procedure was suggested by the practice followed at de Bilt, where the " character " 

 figures supplied by the different observatories are dealt with. Supposing data to be 

 supplied by, say, 40 observatories, the 40 figures assigned to any one day are summed 

 and the mean taken to the nearest O'l, and the result is accepted as an international 

 measure of the amount of magnetic disturbance on the day in question. 



2. The " character" figures in S.M. were based on the curves of only one station, 

 Kew ; they were assigned by a single individual, myself ; and they referred to one 

 period of years, 1890 to 1900. I have thus thought it desirable to repeat the 

 investigation for a second period of years, 1906 to 1911, making use of the inter- 

 national "character" figures published at de Bilt. 1906 was the earliest year for 

 which international figures existed, and 1911 was the latest for which these figures 

 were complete when the present enquiry commenced. As before, five days were 

 selected for each month ; but they were selected solely by reference to the international 

 lists, being the five days of highest " character " figures in each month. When, as 

 occasionally happened, there was a possible choice between two or more days for the 

 last place on the monthly list of five, the criterion applied was that the selected days 

 should, if possible, be consecutive. I had had occasion some years ago, before the 

 present enquiry was even thought of, to select the five most disturbed days of each 

 month of the years 1906 to 1909, and had made use of the above criterion. There 

 seemed no reason to discard the old list, or to follow a different principle when dealing 

 with 1910 and 1911. My experience when forming the first list had led me to regard 

 five as a happy choice for the monthly total of disturbed days. A considerably 

 smaller number, such as one or two a month, gave too few days to eliminate 



