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LIX. On the Cause of the Solar-Diurnal Variations of Ter- 

 restrial Magnetism. By Professor Balfour Stewart, 

 LL.JD., F.R.S.* 



IN attempting to account for the Solar-diurnal inequalities 

 of Terrestrial Magnetism, scientific speculators have 

 ranged over the whole field of likely explanations. Some of 

 them think that they have now finally driven the truth into a 

 corner, and are in hopes of ultimately securing the object of 

 their pursuit. 



I shall begin by enumerating the chief hypotheses which 

 have been proposed. 



(1) There is, first, that of the direct magnetic action of the 

 Sun upon the Earth. There are, however, many serious 

 objections to this hypothesis. One of these is the great 

 difficulty attending the supposition that the Sun is capable of 

 acting as a powerful magnet, since terrestrial analogies would 

 lead us to infer that matter at the high temperature of the 

 Sun is quite incapable of possessing magnetic properties. 

 Again, we have strong evidence that changes in the range of 

 the daily variation of magnetic declination lag behind corre- 

 sponding solar changes in point of time. This kind of 

 behaviour is apparently inconsistent with direct magnetic 

 action, and points rather to some indirect influence probably 

 caused by the radiant energy of the Sun, inasmuch as the 

 changes and turning-points of such indirect influences due 

 to radiation are well known to lag in respect of time behind 

 the corresponding changes and turning-points in their cause. 

 Finally, Dr. Lloyd and Mr. C. Chambers t have both shown 

 that direct solar magnetic action will not account for the 

 peculiarities of the diurnal magnetic variation. We may 

 therefore perhaps dimiss this hypothesis. 



(2) The next hypothesis which I shall introduce is that of 

 Faraday. Reasoning from his discovery that oxygen is 

 magnetic and becomes weaker in its power when heated, he 

 concludes that when the Sun heats a certain portion of the 

 Earth's atmosphere, this renders that portion less magnetic 

 than those other portions not subjected to solar influence. 

 This kind of action is thus seen to be equivalent to a displace- 

 ment by means of solar influence of the magnetic matter of 

 the Earth, and hence implies a corresponding displacement 

 of the lines of magnetic force. Now there is unques- 

 tionably truth in this hypothesis, but yet there is very great 



* Communicated by the Physical Society : read April 10, 1886. 

 t Proc. Royal Irish Academy, Feb. 22, 1858, and Phil. Trans. 1863. 



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