1821.] Prof . Oersted on Electro-magnetism. 335 



not the same upon the earth and the sea, and even differs accord- 

 ing to the elevation of the country above the level of the sea. It 

 is even possible that the different conducting power of different 

 parts of the globe, particularly of land and sea, may possess great 

 influence. It appears then that the new discoveries do not as 

 yet furnish us with new facts sufficiently developed, to be useful 

 in mathematical researches upon the situation of the magnetic 

 poles of the earth. It is nevertheless to be wished that JVL 

 Hansteen, who has displayed so much solid learning in his 

 researches into the magnetism of our globe (undertaken when he 

 could not have recourse to the electro-magnetic discoveries), 

 would resume his labour with the means which natural philoso- 

 phy now offers. I shall confine myself to proposing some ideas 

 to those who are willing to undertake a deeper examination of 

 this subject. 



According to the manner in which magnetism is produced on 

 the globe S z Nz, the strongest magnetic tension ought to occur 

 in a circle round the ends $ and n of the axis s n. We have 

 hitherto supposed that the limits of the electro-magnetic belt 

 were throughout equally distant from the poles of the earth. But 

 there ia reason to suppose that the electro-magnetic effect of the 

 sun is but weak in those places which are covered with ice and snow 

 during a great part of the year. It is then very likely that the 

 electro-jnagnetic belt is very nearly parallel with the isothermal 

 line of 0°. The form of the electro-magnetic belt determines 

 also the form of the curve in which the greatest intensity of 

 magnetism occurs round the poles of the globe s z n z. But the 

 points of this line which are nearest to us would act most 

 strongly upon our magnetic needles, and would appear as mag- 

 netic poles. 



It must be confessed thai in these conclusions we cannot 

 always support ourselves upon evident principles ; but I will, 

 however, cite, as a remarkable confirmation, that the two mag- 

 netic poles, indicated by M. Hansteen, in the northern hemi- 

 sphere are under the same meridian as the celebrated Humboldt, 

 (who has rendered such great services to natural sciences), places 

 the greatest concavity, that is to say, the greatest polar distance, 

 from his isothermal line of 0°. I likewise remember to have 

 heard M. Hansteen remark, several years since, that the mag- 

 netic poles are distinguished by extreme cold. We are now speak- 

 ing of the northern magnetic poles, as to the southern hemisphere, 

 we are in possession of too small a collection of facts to fix the 

 isothermal line. 



The annual and daily variations of the magnetic needle are inti- 

 mately connected with the relation of the earth to the sun, but they 

 do not appear to depend upon any variation in the intensity of the 

 magnetism of the interior of the earth, by the electro-magnetism 

 which the sun produces ; for these variations do not occur upon 

 different parts of our globe at the same time, in such a manner 



