PEOGEESS OF MAGNETIC THEOEY. 225 



the first place, that vulgar opinion of recent writers concerning mag- 

 netic mountains, or a certain magnetic rock, or an imaginary pole at a 

 certain distance from the pole of the earth." For, he adds, " we learn 

 by experience, that there is no such fixed pole or term in the earth for 

 the variation." Gilbert describes the whole earth as a magnetic globe, 

 and attributes the variation to the irregular form of its protuberances, 

 the solid parts only being magnetic. It was not easy to confirm or 

 refute this opinion, but other hypotheses were tried by various writers ; 

 for instance, Halley had imagined, from the forms of the lines of 

 equal variation, that there must be four magnetic poles ; but Euler 14 

 showed that the " Halleian lines" would, for the most part, result 

 from the supposition of two magnetic poles, and assigned their posi- 

 tion so as to represent pretty well the known state of the variation 

 all over the world in 1744. But the variation Avas not the only phe 

 nomenon which required to be taken into account ; the dip at differ- 

 ent places, and also the intensity of the force, were to be considered 

 "We have already mentioned M. de Humboldt's collection of observa- 

 tions of the dip. These were examined by M. Biot, with the view 

 of reducing them to the action of two poles in the supposed terres- 

 trial magnetic axis. Having, at first, made the distance of these 

 poles from the centre of the earth indefinite, he found that his for- 

 mulae agreed more and more nearly with the observations, as the 

 poles were brought nearer ; and that fact and theory coincided tolera- 

 bly well when both poles were at the centre. In 1809, 15 KrafFfc sim- 

 plified this result, by showing that, on this supposition, the tangent 

 of the dip was twice the tangent of the latitude of the place as mea- 

 sured from the magnetic equator. But M. Hansteen, who has devoted 

 to the subject of terrestrial magnetism a great amount of labor and 

 skill, has shown that, taking together all the observations which we 

 possess, we are compelled to suppose four magnetic poles ; two near 

 the north pole, and two near the south pole, of the terrestrial globe ; 

 and that these poles, no two of which are exactly opposite each other, 

 are all in motion, with different velocities, some moving to the east 

 and some to the west. This curious collection of facts awaits the 

 hand of future theorists, when the ripeness of time shall invite them 

 to the task. 



[2nd Eel] [I had thus written in the first edition. The theorist 

 who was needed to reduce this accumulation of facts to their laws, 



14 Ac. Berlin, 1757. 15 Enc. Met. p. 742. 



VOL. II. 15. 



