TERRESTRIAL MAGNETISM. 101 



at the Portuguese colony of Mossamedes (lat. 15 S.), 

 has its concave summit in the middle of the ocean in long. 

 18 W., and touches the Brazilian coast in 20 S. lat. 

 Future observations will show more clearly whether there 

 may not also be a region of comparatively weak magnetic 

 force (0-97 of the arbitrary scale) to the north of the 

 equator in 10 to 12 N. lat., and about 20 to the east of 

 the Philippines. 



According to the data at present possessed, I believe I 

 have little to alter in the ratio given in Vol. I. of the weakest 

 to the strongest terrestrial magnetic force. It falls between 

 1 : 2J and 1 : 3, but nearest to the latter; some 

 diversity of statement being caused by alterations having 

 been rather arbitrarily made, sometimes in the minima only, 

 and sometimes in both minima and maxima at once.( 107 ) 

 Sabine( 108 ) has the great merit of having first called atten- 

 tion to the importance of the " dynamic equator," (the curve 

 of least intensity of the magnetic force). " This curve, which 

 undulates in its progress round the globe, connects the 

 points in each geographical meridian at which the earth's 

 magnetic force is least, the force increasing everywhere in 

 receding from it on either side towards the higher latitudes 

 of the two hemispheres. It marks the physical separation 

 between the two magnetic hemispheres more decidedly than 

 does the line of no dip, on which the direction of the 

 magnetic force is perpendicular to the direction of gravity. 

 For the theory of magnetism all that relates to the force 

 has a more immediate bearing than what relates to the 

 direction of the needle, either in the horizontal or vertical 

 plane ; those planes, although necessarily used by us both 

 in observation and discussion, not having in themselves 

 any direct relation to magnetism. The inflexions of the 



