yb TERRESTRIAL MAGNETISM. 



American focus is the more secure, particularly in respect to 

 latitude : " the longitude," it is said, " is probably rather 

 too westerly ;" it places the oval which encloses the stronger 

 of the two northern foci in the meridian of the western end 

 of Lake Superior. We are indebted for this determination 

 to the important land expedition in 1843 of Captain 

 Lefroy, of the British Eoyal Artillery, formerly Director of 

 the St. Helena, and since of the Toronto, Magnetic Obser- 

 vatory. " The junction of the two loops of the lemniscate 

 appears to be situated north-east of Behring's Straits, some- 

 what nearer to the Asiatic than to the American focus." 



When, in 1802, on the Peruvian chain of the Andes 

 (between Micuipauipa and Caxamarca, in lat. 7 2 S. and 

 long. 81 8' W. from Paris), I intersected the magnetic 

 equator or line on which the inclination is 0, and when I 

 found the magnetic force increase as I proceeded either to 

 the north or to the south from this remarkable spot, there 

 being at that time, and long afterwards, an entire absence 

 of all other points of comparison, I was led by an erroneous 

 generalisation of these, the only data which observation 

 then presented, to believe that the earth's magnetic force 

 would be found to increase continuously from the magnetic 

 equator to each of the magnetic poles, or points of 90 

 inclination, which I thought were probably also the points 

 of greatest terrestrial magnetic force. When we come for 

 the first time on the trace of a great natural law, the views 

 first taken almost always require some subsequent rectifica- 

 tion. Sabine( 95 ) has shown, by his own observations (made 

 through an extensive range of latitude, in the years 1818 to 

 1822), and by a careful combination of these with those of 

 many other observers (experiments of vibration with vertical 



