94 



COSMOS. 



of tolerably low intensity (0-97) lying north of the equator 

 (10 to 12 lat.), and about 20 east of the Philipines, is a 

 question that must be left for future investigations to eluci- 

 date. 



I do not think that the ratio which I formerly gave of the 

 weakest to the strongest terrestrial force requires much mod- 

 ification in consequence of later investigations. This ratio 

 falls between 1:2^ and 1 : 3, being somewhat nearer to the 

 latter number, and the difference of the data* arises from the 

 circumstance that in ^ome cases the minima alone, and in 

 others the minima and maxima together, have been altered 

 somewhat arbitrarily. Sabinef has the great merit of having 

 first drawn attention to the importance of the dynamic equa- 

 tor, or curve of least intensity. " This curve connects the 

 points of each geographical meridian 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 

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

 Cape of Good Hope." 



* 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 traveler (5-35), with the 

 highest value ftr 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 Proceedings 

 of the Meeting at Liverpool, p. 56. 



t See Contributions to Terrestrial Magnetism, No. vii., p. 256. 



