TEREESTEIAL MAGNETISM AT KEW OBSEEVATOEY. 
97 
phenomena of different years shown in Tables VI. and VIII., it is clear that other 
important influences have to be considered. 
These other influences would appear to have sometimes opposed the influence in 
virtue of which a sunspot pulse is followed by a magnetic pulse, and so long as the 
nature of the other influences and the various inter-relationships are unknown, it 
would be unsafe to assume that years of sunspot maximum always behave in the same 
way as 1892 to 1894. 
§ 20. One of the possibilities suggested by Tables V. and VII. was that there might 
be a periodic fluctuation in magnetic properties, which was in phase with sunspot 
variations to a much greater extent in some years than in others. Various such 
periods have in fact been advanced by earlier investigators. A 26-day period has 
been suggested by several magneticians, including Hornstein and Broun. Dr. Ad. 
Schmidt, of Potsdam, claims to have discovered that a large proportion of magnetic 
storms of the very largest kind are separated by intervals which are multiples of 
29‘97 days. In 1904-5, Mr. E. W. Maunder,'^ in two important papers, discussing 
the magnetic storms recorded at Greenwich from 1848 to 1903, claimed to have 
discovered a period of 27'27 5 days, corresponding to the time of rotation of the sun¬ 
spot zones on the sun. The validity of Mr. Maunder’s claim to have established a 
period was supported by the present Astronomer Boy alt amongst others, on arguments 
based on the mathematical theory of probability. 
I had myself J occasion, at the instance of the Editor of ‘ Terrestrial Magnetism,’ to 
read carefully and criticise the first of Mr. Maunder’s papers, dealing with magnetic 
storms from 1888 to 1903. The result left me undecided whether Mr. Maunder had 
established his case. One important point in favour of his contention was that 
practically the same periodic time had been deduced from a study of magnetic storms 
at Toronto by Mr. Arthur Harvey,§ he and Mr. Maunder being ignorant of each 
other’s work. A second fact in its favour was that a list of 125 magnetic storms at 
Kew between 1890 and 1900, got out by myself in an absolutely unprejudiced way, 
gave 21 intervals of from 25 to 28 days, while intervals of from 21 to 24 days and 29 
to 32 days numbered respectively only 7 and 9. I experienced, however, a difficulty 
as to the proper mathematical basis for applying a probability calculation. As it so 
happened, the standard of disturbance accepted as defining a magnetic storm was such 
that the average interval between successive Greenwich storms from 1888 to 1903 was 
29 days, and that between successive Kew storms from 1890 to 1900 was 28 days, 
both being intervals undesirably close to the supposed period. Another difficulty was 
that the times of commencement of the storms, from which Mr. Maunder derived his 
intervals, had an extraordinarily marked diurnal period, showing that they were 
* Eoyal Astronomical Society’s ‘ Notices,’ vol. 65, pp. 2 and 538. 
t ‘ The Observatory,’ vol. 28, 1905, p. 176. 
I ‘ Terrestrial Magnetism,’ vol. 10, 1905, p. 9. 
§ ‘ Trans. Can. Inst.,’ 1898-99, p. 345, &c. 
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