MAGNETIC INTENSITY. 89 



curves, we shall find, in considering the distribution of the 

 magnetic force in each hemisphere, that there are two points, 

 or foci, of the maxima of intensity, a stronger and a weaker 

 one, lying at very unequal distances both from the poles of 

 rotation and the magnetic poles of the earth. Of these four 

 terrestrial points the stronger, or American, is situated in 

 the northern hemisphere,* in 52° 19' N. lat. and in 92° W. 

 long. ; while the weaker, which is often called the Siberian, 

 is situated in 70° N. lat. and in 120° E. long., or perhaps a 

 few degrees less to the eastward. In the journey from Par- 

 schinsk to Jakutsk, Erman found, in 1829, that the curve 

 of greatest intensity (1*742) was situated at Beresowski Os- 

 trow, in 117° 51' E. long, and 59° 44' N. lat. (Erman, 

 Magnet Beob., s. 172-540 ; Sabine, in the Phil. Transact, 

 for 1850, pt. i., p. 218). Of these determinations that of 

 the American focus is the more certain, especially in respect 

 to latitude, while in respect "to longitude it is probably 

 somewhat too far west." The oval which incloses the stron- 

 ger northern focus lies, consequently, in the meridian of the 

 western end of Lake Superior, between the southern extrem- 

 ity of Hudson's Bay and that of the Canadian lake of Win- 

 nipeg. We owe this determination to the important land 

 expedition, undertaken in the year 1843, by Captain Lefroy, 

 of the Royal Artillery, and formerly director of the Magnetic 

 Observatory at St. Helena. " The mean of the lemniscates 

 which connect the stronger and the weaker focus appears to 

 be situated northeast of Behring's Straits, and somewhat 

 nearer to the Asiatic than to the American focus." 



When I crossed the magnetic equator, the line on which 

 the inclination =0, between Micuipampa and Caxamarca, in 

 the Peruvian chain of the Andes, in the southern hemisphere, 

 in 7° 2' lat. and 78° 48' W. long., and when I observed that 

 the intensity increased to the north and south of this remark- 

 able point, I was led, from an erroneous generalization of 

 my own observations, and in the absence of all points of 

 comparison (which were not made till long afterward), to 

 the opinion that the magnetic force of the earth increases 

 uninterruptedly from the magnetic equator toward both 

 magnetic poles, and that it was probable' that the maximum 

 of the terrestrial force was situated at these points, that is 



* In those cases in which individual treatises of General Sabine 

 have not been specially referred to in these notes, the passages have 

 been taken from manuscript communications, which have been kind- 

 ly placed at my disposal by this learned physicist. 



