AND OF TERRESTRIAL MAGNETISM. 247 



accidental features, unless a much larger number of years were available. On the 

 other hand, if one took as many as ten days, there would in most months be several 

 days competing for the last place on tin- list, and during magnetically quiet times 

 m.iiiy of the days occurring in tin- monthly choice would have represented quiet 

 rather than disturbed conditions. 



The present p:i|-r is not confined to the period 1906 to 1911, but utilises as well 

 my original data for 1890 to 1 900 for the investigation of various points not considered 



inS.M. 



^ :;. Tin- first step was to make sure that the period of approximately 27- days was 

 confirmed by the international "character" figures from 1906 to 1911. The mean 

 results obtained for the individual years from 5 days before to 30 days after the 

 representative day n of large disturbance are given in Table I. The entries represent 

 the mean international " character " figure The last column gives for comparison the 

 mean "character" figure for all days of the year. In the case of 1911, December 

 was excluded, so as to keep all the days dealt with within the six years. The results 

 were really taken out to three decimal places, and these more exact values were used 

 in calculating some of the later results in the paper. 



4. Before discussing the main question, some phenomena in Table I. call for 

 remark. The entries in column n and the means from all days show but little 

 variation from year to year, and the natural inference would be that the six years 

 were almost equally disturbed. The phenomena, however, is I believe largely due to 

 another cause. The international data are published quarterly. Thus the man 

 whose duty it is to assign " character " figures at any observatory naturally deals with 

 the curves of not more than three months at a time. In most cases, doubtless, he 

 has a desire to maintain something like a uniform standard ; but unless his verdict is 

 based on the exact measurement of some definite quantity, such as the daily range, 

 he is inevitably much influenced by the accident of whether the months he is dealing 

 with are quiet or disturbed. One of the leading objects is the discrimination 

 between the days of each individual month, and if " O's " are given to nearly all the 

 days of a very quiet month, there is no adequate discrimination. The natural 

 tendency is thus to assign a " 1 " in quiet months to days which in highly-disturbed 

 months would naturally get a "0." 



5. Another point to bear in mind is that highly disturbed conditions are seldom 

 confined to a single day, and not infrequently extend over three or four consecutive 

 days or even more. Not infrequently three or even four of the five most disturbed 

 days of the month were consecutive. In February, 1907, the whole five were con- 

 secutive days, and in March and April, 1910, seven of the ten selected disturbed 

 days were consecutive. This explains why the " character" figures for days nl and 

 n+1 in Table I. invariably are next in magnitude to those for days n. But the next 

 highest figure, it will be seen, occurs on day n + 26 (once), n + 27 (four times), or 

 (once). 



