TERRESTRIAL MAGNETISM. 95 



America in the years 1798 1804. Still earlier results on 

 the earth's magnetic force had been obtained by my friend 

 De Rossel in 1791 and 1794 in the Indian Seas, but they 

 were not printed until four years after my return from Mexico. 

 In 1829 I had the advantage of being enabled to add to my 

 determinations of the magnetic force and inclination, so as 

 to make them extend over 188 of longitude (or more than 

 half the circumference of the globe), viz. from the shores of 

 the Pacific Ocean eastward to the Siberian frontier of the 

 Chinese territories, two-thirds of the interval being across 

 the interior of continents. The differences of latitude were 

 from 60 N. to 12 S., or 72. 



If, in examining the distribution of the magnetic force over 

 the surface of the earth, we consider carefully the direction of 

 the successive isodynamic lines (or curves of equal magnetic 

 force) enclosing each other, and proceed from the outer or 

 weakest force to the inner curves of successively stronger 

 force, we find in each hemisphere, at very unequal distances 

 both from the poles of the earth and the magnetic poles, 

 two points, or, as they have been called, " foci," of maximum 

 force, one stronger and the other weaker. Of these four 

 points on the earth's surface, we find, taking the northern 

 hemisphere first, ( 94 ) the strongest (the American focus) in 

 lat. 52 19', and long. 91 ^58 W. ; and the weaker (often 

 termed the Siberian focus) in lat. 70 N., long. 120 E., 

 or perhaps a few degrees less easterly. Erman, in 1829, in 

 travelling from Parchinsk to lakutzk, found the curve of 

 greatest intensity (1*742) near Beresowski Ostrow, in long, 

 117 53' E., lat. 59 44' N.) ; Erman, Magnet. Beob. 

 S. 172 and 540 ; Sabine, in the Phil. Trans, for 1850 

 (Pt. i. p. 218). Of these two determinations, that of the 



