2 Qn M. Hansteen's recent Magnetic 



by the government of the United States, for scientific re- 

 searches in the southern hemisphere, I cannot anticipate a more 

 favourable opportunity of turning to good account the infor- 

 mation of which M. Hansteen has so liberally made me the 

 depository. Since analogy would lead us to expect that a 

 corresponding system of magnetism prevails in the two hemi- 

 spheres of our globe, a knowledge of the arrangement of the 

 system in the northern hemisphere may prove an important 

 guide and direction for corresponding researches in the south- 

 ern ; whilst the example of M. Hansteen's undertaking may 

 stimulate, and his success is well calculated to encourage, those 

 who are about to enter on a career honourable alike to them- 

 selves and to the government under whose instructions they 

 are employed. 



For some years past it has been the opinion of several per- 

 sons who have attentively considered the subject, that a know- 

 ledge of the general system of the magnetism of our globe is 

 more likely to be attained by experiments on the relative 

 intensity of the magnetic attraction in different parts of the 

 earth's surface, than by observations on the Dip, or Variation, 

 of the needle. In conformity with this opinion, M. Hansteen 

 (without, however, neglecting to observe on all occasions the 

 three phenomena conjunctively) has applied himself especially 

 to trace the lines connecting those places on the globe, where 

 a needle freely suspended in the magnetic direction, and drawn 

 a certain number of degrees from rest, is found to make an 

 equal number of vibrations around its point of rest in a given 

 time. It was to be expected that these lines of equal inten- 

 sity would arrange themselves systematically round the point 

 or points in each hemisphere where the intensity was greatest : 

 and, on the supposition that two such points would be found 

 opposite to each other on the globe, one in the northern and the 

 other in the southern hemisphere, that the isodynamic lines 

 would form parallel circles, analogous to those of geographical 

 latitude, progressively diminishing in intensity from the two 

 points of maximum or poles, to the boundary circle of the two 

 hemispheres, which circle, following the same analogy, might 

 receive the appellation of the Magnetic Equator. Such was 

 in fact the system, which, until the decisive discoveries which 



