TERRESTRIAL MA.GNETISM 99 



changes which future centuries shall manifest, changes 

 depending, it may be, on the influence produced by the mag- 

 netic force of the sun on the magnetism of the earth. 



In the northern hemisphere the determination of the force 

 at the strongest or Canadian focus (lat. 52 19', long. 

 92 W.), by Lefroy, is the most satisfactory. It is expressed 

 in the relative scale by 1*878, the force in London being 

 1-372; and in the absolute scale by 14*21( 100 ). Even so 

 far south as New York, in lat. 40 42', Sabine had found 

 the magnetic force not much less than at the maximum, 

 viz. 1-803. At the weaker northern focus, the Siberian 

 one (lat. ? 70, long. 120 E.), the force was found, in the 

 relative scale, by Erman, 1'74, and by Hansteen, 1'76; or, 

 expressed in absolute measure, 13*3. The Antarctic Expe- 

 dition of Sir James Ross has taught us, that the difference of 

 force between the two foci of the southern hemisphere is 

 probably less than between the two foci of the northern 

 hemisphere; and that the intensity of the force is itself greater 

 at each of the southern foci than at either of the northern 

 foci. The intensity of the force at the stronger southern 

 focus (lat. 64 S., long. 137 30' E.) is in the relative 

 scale at least 2*06, ( 101 ) in the absolute scale 15'60 : at the 

 weaker southern focus,( 102 ) lat. 60 S., long. ? 125 W., 

 still, according to Sir James Ross, it is in the relative scale 

 1-96, and in the absolute 14' 90. The greater or less 

 distance apart of the two foci in the same hemisphere is 

 recognised as an important element of their individual 

 strength and of the whole distribution of the magnetic force 

 at the surface of the earth. " Although the foci of the 

 southern hemisphere present a strikingly greater magnetic 

 force (in absolute measure 15*60 and 14*90) than those of 



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