NOTES. ]v 



in the Polar Sea, to the North or North east of Bering's Strait, and consequently 

 nearer to the Siberian than to the American focus. The corresponding 

 ph&nomena of the southern hemisphere are not yet determined with an equa 

 precision, but appear to have the same general characteristics. The two major 

 foci, one in the northern, the other in the southern hemisphere, are not at 

 opposite points of the globe to each other, nor are the two minor foci. Sym- 

 metry as regards geographical distribution is also departed from in another 

 lespect: the foci in each hemisphere are not separated from each other by an 

 equal number of degrees of geographical longitude ; they are nearer to each 

 other in the southern than in the northern hemisphere. 



It is well known that in the extra-tropical latitudes of the northern hemi- 

 sphere, the forces which attract the north end of a magnet (so called, because 

 in the greater part of the accessible portions of the globe it is that end 

 of the magnet which is directed towards the north), and repel the south 

 end, preponderate ; and that conversely, in the extra-tropical parts of the 

 southern hemisphere, the forces which attract the south end and repel the 

 north predominate. At both the foci of the northern hemisphere the pre- 

 dominance is of the forces which attract the north end and repel the south ; 

 and at both the foci in the southern hemisphere the converse is the case. 

 The line which separates the preponderance of the northern from that of 

 the southern attracting force, is a line drawn through the points in each 

 meridian of the globe where the intensity of the force is weakest on that 

 meridian. Every where on the north side of this line the preponderance of 

 the force attracting the north end of the magnet, and repelling the south, 

 increases ; and every where on the south side 6f the line the preponderance 

 of the force attracting the south end, and repelling the north, increases. 

 This line is not one of those which has been characterised or referred to 

 by M. de Humboldt, but it is an important one in the view which it enables 

 us to take of the magnctical relations of different portions of the globe. 

 Its inflections are various, as will easily be imagined Avhen it is considered 

 that they take place in conformity with forces which produce four points 

 of maximum on the surface of the globe unsymmetricaUy distributed, and 

 also unsymmetrical in respect to the intensity of the force at each. The 

 most remarkable inflection is the large convexity towards the south, which 

 advances into the southern Atlantic nearly to the latitude of 20 S., in 

 consequence of the wide separation which exists in that quarter between 

 the major and minor foci of the southern hemisphere. 



The measurement of the magnetic force in parts of an absolute scale, 

 suggested by M. Poisson, and brought into use by M. Gauss, has added greatly 



