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Sun-spots and Terrestrial Magnetism. [Dec. 18 



Another recent and able paper bearing on the subject appears in the 

 last published volume of the French Bureau Meteor ologi que, which 

 has just come into my hands. The author, Mr. Alfred Angot, has 

 anticipated me in applying a formula of type (1) to the individual 

 months of the year ; but he treabs of the amplitude, not of the diurnal 

 range as a whole, but of that of the coefficients of the several terms of 

 the Fourier's series into which the diurnal inequality can be analysed. 

 The paper treats only of the declination — dealing with data from 

 ordinary days at Pare St.-Maur, Greenwich and Batavia — but the 

 author expresses his intention of considering in the future the 

 horizontal force. 



A special feature of the present investigation is that the magnetic 

 data are derived exclusively from magnetic quiet days. This suggests 

 at once a query and a criticism, a query as to why one did not employ 

 corresponding sun-spot data confined to the magnetically quiet days, 

 a criticism that as the two sets of data employed clo not absolutely 

 correspond, the comparison actually made may be misleading. 



As to the query : Wolfer, it is true, publishes at regular intervals in 

 the ' Met. Zeitschrif t ' provisional sun-spot frequencies for each day. 

 These figures are, however, presumably inferior in certainty to the final 

 figures he has embodied in his table after consulting all available 

 sources of information. The vital consideration, however, is that at 

 certain seasons of the year there are a number of days for which, owing 

 to the absence of observations, Wolfer has no provisional sun-spot 

 data. With information lacking for two or three out of the five quiet 

 days of a month there would have been a very undesirable amount of 

 uncertainty. As to the criticism, it would be difficult to meet it if it 

 could be held that the enhanced magnetic activity existing at the 

 earth's surface at times of sun-spot maxima is due directly to electrical 

 disturbances in the sun, each disturbance being limited to regions 

 where sun-spots exist, and only those disturbances being effective which 

 happen to be at the moment on the half of the sun visible from the 

 earth. At present I shall only mention the following fact : — I had 

 monthly sun-spot frequencies calculated from Wolfer's provisional figures, 

 employing only the five " quiet " days selected for each month by the 

 Astronomer Royal. The mean sun-spot frequency thence deduced for 

 the eleven years (1890 to 1900) differed from the corresponding result 

 given by all Wolfer's days by less than one-fifth of 1 per cent. It is 

 hardly necessary to point out that this fact has an important bearings 

 not only on the point immediately under consideration, but also on 

 the further question as to the true nature of the connection between 

 sun-spots and magnetic storms. 



