MAGNETIC INTENSITY. 95 



what nearer to the latter number, and the difference of 

 the data 7 arises from the circumstance that in some cases 

 the minima alone, and in others the minima and maxima 

 together, have been altered somewhat arbitrarily. Sabine 8 

 has the great merit of having first drawn attention to the 

 importance of the dynamic equator, or curve of least intensity. 

 " This curve connects the points of each geographical meri- 

 dian at which the terrestrial intensity is the smallest. 

 It describes numerous undulations in passing round the 

 earth, on both sides of which the force increases with the 

 higher latitudes of each hemisphere. It in this manner 

 indicates the limits between the two magnetic hemispheres 

 more definitely than the magnetic equator, on which the 

 direction of the magnetic force is vertical to the direction of 

 gravity. In respect to the theory of magnetism, that which 

 refers directly to the force itself is of even greater importance 

 than that which merely refers to the direction of the needle, 

 its horizontal or vertical position. The curves of the 

 dynamic equator are numerous, in consequence of their 

 depending upon forces, which produce four points (foci) of 

 the greatest terrestrial force, which are unsymmetrical and 

 of unequal intensity. We are more especially struck in these 

 inflections with the great convexity in the Atlantic Ocean 

 towards the South Pole, between the coasts of Brazil and the 

 Cape of Good Hope." 



Does the intensity of the magnetic force perceptibly 

 decrease at such heights as are accessible to us, or does it 

 perceptibly increase in the interior of the earth ? The pro- 

 blem which is suggested by these questions is extremely 



7 We obtain 1:2.44 on comparing in the absolute scale St. Helena, 

 which is 6.4, with the focus of greatest intensity at the south pole, 

 which is 15.60, and 1:2.47 by a comparison of St. Helena with the 

 higher southern maximum of 15.8, as given in the Admiralty Manual, 

 p. 17, and 1 : 2.91 by a comparison in the relative scale of Erman's ob- 

 servation in the Atlantic Ocean (0.706), with the southern focus (2.06) ; 

 indeed, even 1:2.95, when we compare together in the absolute scale 

 the lowest value given by this distinguished traveller (5.35), with the 

 highest value for the southern focus (15.8). The mean resulting ratio 

 would be 1 :2.69. Compare for the intensity of St. Helena (6.4 in the 

 absolute, or 0.845 in the arbitrary scale), the earliest observations of 

 Fitzroy (0.836), Phil. Transact, for 1847, pt. i, p. 52, and Proceeding of 

 tfie Meeting at Liverpool, p. 56. 



8 See Contrib. to Terrestr. Magnetism, No. vii, p. 256. 



