MAGNETIC INTENSITY. 69 



over were instituted at the suggestion of Borda, are those 

 which I made during my voyage to the tropical regions of 

 the New Continent between the years 1798 and 1804. The 

 results obtained at an earlier date (from 1791 to 1794), 

 regarding the magnetic force, by my friend de Rossel, in the 

 Indian Ocean, were not printed till four years after my 

 return from Mexico. In the year 1829 I enjoyed the advan- 

 tage of being able to prosecute my observations of the mag- 

 netic intensity and inclination over a space of fully 188 

 of longitude from the Pacific eastwards as far as the Chinese 

 Dzungarei, two-thirds of this portion of the" earth's surface 

 being in the interior of continents. The differences in the 

 latitudes amounted to 72 (namely, from 60 N. to 12 

 S. Lat.). 



When we carefully follow the direction of the closed 

 isodynamic lines (curves of equal intensity), and pass from 

 the external and weaker to the interior and gradually stronger 

 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 94 in 52 19' N. lat. and in 92 W. 

 long., whilst 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 Ostrow 

 in 117 51 ' E. long, and 59 44' K lat. (Erman Magnet. Beob. 

 s. 172540; Sabine, in the Phil Transact, jor 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 stronger northern 

 focus lies, consequently, in the meridian of the western end 

 of Lake Superior, between the southern extremity of Hud- 



94 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 kindly 

 placed at my disposal by this learned physicist. 



