70 Professor Hansteen on the Magnetic Poles of the Earth. 



In answer to the question which naturally arises respecting 

 the cause of these remarkable phenomena, Professor Hansteen 

 makes the following observation : It is possible that the illu- 

 mination and heating of the earth, during one revolution about 

 its axis, may produce a magnetic tension, as well as it pro- 

 duces the electrical phenomena, and that the change of posi- 

 tion in the magnetic axis may be explained from a change of 

 position in the earth's axis to its orbit. 



Professor Hansteen next proceeds to show how the changes 

 in the variation and dip of the needle may be explained by the 

 motion of the magnetic poles ; and he begins with the obser- 

 vations made at Paris, where the variation was as follows : 



Now it appears that, in 1 580, the Siberian pole n was about 

 40* east of Greenwich, or north of the White Sea, while the 

 North American pole N was about 136* west of Greenwich, or 

 about 30° east of Behring's Straits. The pole n, therefore, 

 lay nearer Europe than now, and the pole N was more remote. 

 Hence the former exercised a predominant action, and the 

 needle turned towards the east. In the mean time, the pole 

 n withdrew itself towards the Siberian Ocean, and as N ap- 

 proached Europe, its action increased, and the needle turned 

 westward till 1814, when it reached its greatest declination, 

 and since that time it is evidently returning eastward. On 

 the very same principles we see the reason why the eastern 

 declination was less before 1580. 



The variations of the needle in the Southern hemisphere are 

 explicable in the same way. At the Cape of Good Hope, and 

 in the different bays of the adjoining sea, the variation was 

 easterly in 1605. The following are the variations since the 

 time of Vasco de Gama : 





