312 TWO MAGNETIC POLES. SECT. XXX. 



dip is 90, and two localities where the force forms a 

 center of greatest intensity around which the isodynamic 

 lines arrange themselves. The localities of dip 90 are 

 rather spaces than points : they are the major axes of 

 small ovals on the surface of the sphere ; consequently 

 they are linear rather than circular spaces. The spot 

 where Captain Ross observed the needle so nearly ver- 

 tical in 1831 marks the approximate position of that lo- 

 cality at that epoch. This position is, as Mrs. Som- 

 erville states, about 70 north, and 97 west. The 

 isodynamic centers in the same hemisphere are situ- 

 ated, one in America, the other in Siberia. The ob- 

 servations made anterior to 1837, which are collected 

 and arranged in Colonel Sabine's report to the British 

 Association of that year, gave, when treated by M. 

 Gauss according to the formation of the "Allgemeine 

 Theorie," the American maximum in 55 north and 97 

 west, and the Siberian in 71 north and 116 east. The 

 more recent observations of Messrs. Lefroy and I>ocke, 

 who have traveled in America expressly for the more 

 accurate determination of what appears so important a 

 datum in terrestrial physics, and whose results are at 

 this moment being arranged on a chart on which Colonel 

 Sabine is about to trace the lines of highest intensity in 

 America, show that the center of those curves is yet 

 farther to the southward by some degrees (consequently 

 still more removed from the position where the dip is 

 90) than was supposed in 1837. 



The two maxima of force are not of equal strength : 

 the Siberian is somewhat the weaker of the two. The 

 positions of both undergo secular change, and both in 

 the same direction, viz. to the eastward. The secular 

 change of the weaker or Siberian maximum is far more 

 considerable than that of the other. The secular 

 changes of the isoclinal and isogonic curves correspond 

 with those of the two systems of forces indicated by 

 distinct maxima having unequal movements of transla- 

 tion. The higher isoclinal curves are oval, having their 

 major axes in the line of direction joining the two points 

 of maximum intensity. The general arrangement in the 

 south hemisphere is strictly analogous : but the two 

 centers of force are at this epoch separated by a less in- 



