316 Dr. C. Chree on Deductions from 



§ 10. Supposing a period existent in disturbances, it is 

 obviously desirable to determine it as exactly as possible, if 

 only with a view to ascertaining the true source. At least 

 two previous investigators had, unknown to Mr. Maunder, 

 investigated the matter from practically the same standpoint. 

 How closely similar Mr. Allan Broun's * views were may be 

 seen from his paper in the Phil. Trans, for 1876, p. 400, or 

 from the account given by Balfour Stewart in the Km. Brit. ; 

 but the time which he deduced for the period from the 

 Makerstoun disturbances was a day less than Mr. Maunder's 

 estimate. So again it appears that the Toronto disturbances 

 led Mr. Arthur Harvey t a year or two ago to conclusions 

 almost identical with Broun's and Maunder's, but the period 

 he deduced (27*246 days) differs slightly from Maunder's. 



This all suggests the advantage of some more strictly 

 mathematical method, whether the periodogram method of 

 Prof. Schuster or another, which can not merely assign a 

 value for a period, but also afford a measure of its probability. 

 There is also obviously an advantage to be derived from 

 having a variety of data. The horizontal force at least might 

 well be treated, and quite independently of the declination, 

 and the criteria for selecting disturbances should be varied. 

 Even supposing a fixed minimum range limit assigned, it is 

 at least highly doubtful whether Mr. Ellis's is ideal for an 

 investigation of the kind. His limit is such that in the 

 22 years of Mr. Maunder's first paper the average interval 

 between two successive storms was approximately 29'1 days, 

 while in the 34 years of his second paper it was approximately 

 2 7* 6 days. It does not seem desirable that the average 

 interval should approach so closely to the period it is desired 

 to investigate. There ought under the circumstances — period 

 or no period — to be an exceptionally large number of cases 

 in which storms follow one another at intervals of from 27 to 

 28 days. 



Obviously if Mr. Ellis's lowest limit were much raised the 

 number of storms would become so small as to render it 

 difficult to recognize any 27 to 28 day period however real. 

 Thus, if a fixed limit be adopted, the better alternative would 

 be to lower J Mr. Ellis's limit, though it might be necessary to 



* Mr. Broun accepted apparently three periods, viz., 26, 27*3, and 29v5 

 days, as occurring- in the small regular changes of horizontal force, 

 attributing the former to the sun, the two latter to the moon. But he. 

 found distinct trace in the disturbances only of the 26-day period. 



t Trans. Can. Inst. 1898-9, p. 345 ; Roy. Ast. Soc. of Canada Selected 

 Papers & Proc. 1902-3, p. 74, 1904, p. xiv &c. 



| In some months it is difficult to get more than 5 or 6 days free 

 from very appreciable disturbance. Thus any large reduction in Mr. Ellis's 

 qualifying limit would hardly be compatible with the application of his 

 method. 



