MAGNETIC INTENSITY. 



91 



the neighborhood of the Terre d'Ade'lie, discovered by D'Ur- 

 ville, and therefore in about 67° S. lat. and 140° E. long. 

 He thought that he had approached the other focus in 60° 

 S. lat. and 125° W. long.; but he was disposed to place it 

 somewhat further south, not far from the magnetic pole, and 

 therefore in a more easterly meridian.* 



Having thus established the position of the four maxima 

 of intensity, we have next to consider the relation of the 

 forces. These data can be obtained from a much earlier 

 source, to which I have already frequently referred ; that is 

 to say, by a comparison with the intensity which I found at 

 a point of the magnetic equator in the Peruvian chain of the 

 Andes, which it intersects in 7° 2 / lat. and 78° 48 7 W. long., 

 or, according to the earliest suggestions of Poisson and Gauss, 

 by absolute measurement. f If we assume the Intensity at 

 the above-indicated point of the magnetic equator — 1-000 in 

 the relative scale, we find, from the comparison made be- 

 tween the intensity of Paris and that of London in the year 

 1827 (see page 68), that the intensities of these two cities 

 are 1-348 and 1-372. If we express these numbers in ac- 

 cordance with the absolute scale they will stand as about 

 = 10*20 and 10-38, and the intensity, which was assumed 

 to be 1-000 for Peru, would, according to Sabine, be 7-57 

 in the absolute scale, and therefore even greater than the 

 intensity at St. Helena, which, in the same absolute scale, 

 = 6*4. All these numbers must be subjected to a revision 

 on account of the different years in which the comparisons 

 were made. They can only be regarded as provisional, 

 whether they are reckoned in the relative (or arbitrary) scale 

 or in the absolute scale, which is to be preferred to the for- 

 mer ; but even in their present imperfect degree of accuracy 

 they throw considerable light on the distribution of the mag- 

 netic force — a subject which, till within the last half cen- 

 tury, was shrouded in the greatest obscurity. They afford 



(while crossing the southern isodynnmic ellipse of 200, about midway 

 between the extremities of its principal axis), between the southern 

 latitudes 58° and 64° 26', and the eastern longitudes of 128° 40' and 

 148° 20' (Contrib. to Terr. Maya., in the Phil Transact, for 184G, pt. 

 iii., p. 252). 



* Ross, Voyage, vol. ii., p. 224. In accordance with the instructions 

 drawn up for the expedition, the two southern foci of the maximum 

 of intensity were conjectured to be in 47° S. lat. 140° E. long., and in 

 G0° S. lat. 235 E. long. (vol. i., p. xxxvi.). 



t Phil Transact, for 1850, pt. i., p. 20 L; Admiralty Manual, 1840, 

 p. 1G ; Erman, Magnet. Beob., s. 437-451. 



