PROGRESS OF MAGNETIC THEORY. 63 



the only phenomenon which required to be taken 

 into account; the dip at different places, and alsQ 

 the intensity of the force, were to be considered. 

 We have already mentioned M. de Humboldt's col- 

 lection of observations of the dip. These were 

 examined by M. Biot, with the view of reducing 

 them to the action of two poles in the supposed 

 terrestrial magnetic axis. Having, at first, made 

 the distance of these poles from the center of the 

 earth indefinite, he found that his formulae agreed 

 more and more nearly with the observations, as the 

 poles were brought nearer; and that fact and theory 

 coincided tolerably well when both poles were at 

 the center. In 1809 U , Krafft simplified this result, 

 by showing that, on this supposition, the tangent 

 of the dip was twice the tangent of the latitude 

 of the place as measured from the magnetic equator. 

 But M. Hansteen, who has devoted to the subject 

 of terrestrial magnetism a great amount of labour 

 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, 

 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 dif- 

 ferent velocities, some moving to the east, and 

 some to the west. This curious collection of facts 

 awaits the hand of future theorists, when the ripe- 

 ness of time shall invite them to the task (D). 



14 Enc. Met. p. 742. 



