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XLVII. On the Diurnal Period of Terrestrial Magnetism. 

 By Arthur Schuster, F.R.S* 



FT^HE explanation of the daily variation of the magnetic 

 JL forces observed on the surface of the earth will, in all 

 probability, lead to the explanation of the mysterious connec- 

 tion between solar phenomena and terrestrial magnetism. 

 For the increase in amplitude of the diurnal variation of the 

 horizontal components of magnetic force forms one of the 

 most striking effects accompanying the increase in sunspot 

 activity. The daily variation, then, seems a most important 

 symptom of solar influence, and its investigation becomes a 

 matter of great interest. 



In the remarks which I wrote out for the Report of the 

 Committee appointed by the British Association, for the 

 purpose of considering the best means of comparing and 

 reducing magnetic observations, I pointed out the importance 

 of adopting a suggestion, made already by Gauss, to apply 

 the analysis of surface harmonics to the diurnal oscillations. 

 It is well known that such an analysis would allow us to 

 decide the question whether the immediate cause of the dis- 

 turbance was inside or outside the surface of the earth ; nor 

 can there be two opinions as to the importance of definitely 

 settling that question. At the time I wrote out my sugges- 

 tions, however, it seemed to me, as the causes of the 

 disturbance had their seat in all probability close to the sur- 

 face, whether outside or inside, that we should require a large 

 number of terms in the expansion before we could arrive at a 

 definite result. 



In this I was mistaken ; and it is one of the principal 

 objects of this paper to show that the periodic variations adapt 

 themselves with great facility to the analysis, and that, even 

 with the very limited quantity of material at our disposal, we 

 shall be able to arrive at most important results ; results 

 which within a short time might be made absolutely certain, 

 if additional observations w T ere taken at a few well-selected 

 stations. My results, as far as they go, point definitely to 

 the region outside the surface of the earth as the locality of 

 the periodic cause of the variation. 



It is easy to see that if electric currents parallel to the 

 earth's surface produce any disturbance, we can readily find 

 out whether these currents are outside or inside the earth. 

 As we pass through any current sheet the normal magnetic 



* Communicated by the Author, having been read before the Literary 

 and Philosophical Society of Manchester, January 26, 1886. 



