96 Mr. J. A. Broun on Terrestrial Magnetism. 



that its magnetic poles will not coincide with the poles of rota- 

 tion ; perhaps even the poles may have unequal forces. In such 

 a case, it might be expected that the fact could be determined by 

 our magnetic observations. It happens, however, that the 

 pei'iod of the synodal rotation of the sun (employing the results 

 obtained by Bianchi and Laugier, and given by Sir John Her- 

 schel as the best determination, namely 25'^ 7^ 48"^*) is exactly 

 that of a nodal revolution of the moon. It might therefore be 

 difficult to determine whether any result obtained was not due to 

 the moon's varying position relatively to the plane of the equator. 



The result which I have now obtained from three years' obser- 

 vations near the magnetic equator, it appears to me, is wholly in- 

 dependent of the moon, and is due to the sun's rotation on its axis. 



If we could suppose that the solar magnetic poles are fixed, 

 it might then be possible to detemnine accurately the time of the 

 sun's rotation by means of the movement of our magnets. If, 

 on the other hand, the poles are in motion, as I conceive they 

 are, we shall have to employ another period than 35'^'325, as 

 obtained from the solar spots. The period to be employed will 

 of course be found by careful examination of the observations, 

 and by trial. Such a movement of the solar poles might explain 

 the secular magnetic variations. 



It is not improbable, after all, that some of the results obtained 

 previously, as due to the moon, ave really due to the sun's rotation. 



Supposing that I have proved the sun's magnetic action, it 

 has occurred to me that the fact might be applied to give some 

 ideas for a theory of comets. Sir John Herschel, I believe, has 

 somewhere suggested electricity as the cause which directs the 

 tails of comets. I have looked over the different hypotheses 

 given by M. Arago in the recent edition of his works, where he pro- 

 fesses to omit none ; but no polar hypothesis is to be found there. 



Are not comets formed of magnetic gases ? Is not the tail of 

 the comet due to the directive action of the solar magnet, the 

 curvature of the tail, sometimes seen, being due to the position 

 of the solar magnetic poles relatively to the path of the comet ? 

 Is not the condensation of the comet, when approaching the 

 sun, a phajnomenon similar to those observed by Dr. Faraday and 

 M. PI ticker in their recent researches on the action of the poles 

 of a magnet on certain gases or liquids ? might not a like illustra- 

 tion be given of the varying form of the tufts in the nucleus f ? 



* Outlines of Astronomy, p. 232. 



t Since this was written I find that Bessel considered that the vibrations 

 of the tail of Ilalley's comet, observed by him, indicated the action of a 

 polar force. Arago has made no mention of this hypothesis, aj)parently 

 because he doubted the results on which it was founded {Astronomie Popu- 

 laire, vol. ii. j). 39G). 



