AND OF TERRESTRIAL MAGNETISM. 



24'J 



Taking the means from the six years, the mean "character " figures for days n + 27 

 and n + 28 considerably exceed all others, that for day n + 27 being decidedly the 

 larger. The dost- ivsrmlil.mrr to the results for the epoch 1890 to 1900 in S.M. will 

 be readily recognised on consulting fig. 1. 



o-5 



In 1890 to 1900 the mean character figures for day n, for day n + 27, and for the 

 mean day of the period were respectively 1'51, 0'94 and 070, so that the excess of 

 the " character " figure for day n + 27 over that for the average day was 30 per cent, 

 of the excess for day n. In 1906 to 1911 the corresponding percentage is 27. 



It is unlikely that my personal standard for disturbance when assigning "character" 

 figures to the days of 1890 to 1900 agreed with that of the international list, which 

 represents a compromise of most diverse standards from some forty observatories. 

 Thus the fact that the mean " character " figure for the selected disturbed days of 

 1906 to 1911 was only 87 per cent, of that for the selected disturbed days of 1890 to 

 1900 does not necessarily imply that the second epoch was the quieter of the two. 

 Such, however, was actually the case on the whole, though no year of the later period 

 was as quiet as 1900. 



The two curves of fig. 1 agree in showing no decided trace of any period shorter 

 than 27 days. Other points of resemblance are that the fall subsequent to the 

 maximum during days n + 28 to 71 + 30 is decidedly slower than the rise during days 

 n + 25 to n + 27, and that the pulse centering about day /i + 27 is spread over more 

 days than the primary pulse centering at day n. The latter phenomenon would 

 obviously tend to happen if the period had not always the same length but oscillated 

 slightly about a mean value. 



6. With a view to following up this last idea, I took from the selected disturbed 

 days of the six years all those whose "character" figures were not less than 1'5, the 



VOL. CCXITI. A. 2 K 



