Influence of Terrestrial Magnetism on Iron. 207 



magnetised needle, delicately suspended and brought near to 

 them. The terrestrial globe, therefore, acts as a great mag- 

 net would act whose axis, passing through the centre of the 

 earth, should be situated in the magnetic meridian, and which 

 should have, at the north, a pole contrary to the pole of the 

 needle that is directed to the north ; and at the south an- 

 other pole, in like manner contrary to the pole of the needle 

 that is directed to the south. This hypothesis upon the 

 cause of terrestrial magnetism would also explain the direc- 

 tion of the magnetised needle ; which would also be the 

 result of the attractions exercised by the magnetic poles 

 of the globe upon the contrary poles of the magnetised 

 needle. It would be sufficient, therefore, in order to know 

 the position of the magnetic pole of the earth, situated at the 

 north, to determine carefully the direction of the dipping 

 needle in several different places ; and the intersection of 

 these lines would be the point where the pole in question 

 would be found. It would be necessary to go through the 

 same process on the other side of the magnetic equator, in 

 order to determine the position of the terrestrial magnetic 

 pole situated on the south. But it is found that the direc- 

 tions of the dipping needle, when produced, whether in the 

 boreal or austral hemisphere, do not all intersect exactly 

 in the same point ; which would seem to prove that there is 

 not, therefore, in each hemisphere, a single pole or a single 

 centre of magnetic action. We shall return, in detail, to this 

 interesting subject of terrestrial physics, when we shall be 

 treating upon the numerous observations that have been col- 

 lected upon the different phenomena of terrestrial magnetism, 

 and upon the hypotheses that have been made of its origin 

 and its nature. 



It may not be useless to mention here a denomination still 

 employed in French works, and which owes its origin to the 

 terrestrial theory of the magnet. Setting out from the hypo- 

 thesis that the earth possesses two magnetic poles, and from 

 the principle established by experiment, that poles of the con- 

 trary name attract each other, they have called that pole of the 

 magnetised needle that is directed towards the north, the aus~ 



p2 



