90 COSMOS. 



son's Bay and that of the Canadian lake of Winipeg. 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 lem- 

 niscates which connect the stronger and the weaker focus 

 appears to be situated north-east 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 

 remarkable 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 afterwards), to 

 the opinion that the magnetic force of the earth increases 

 uninterruptedly from the magnetic equator towards both 

 magnetic poles, and that it was probable that the maximum 

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

 to say, where the inclination = 90. When we first strike 

 upon the trace of a great physical law, we generally find that 

 the earliest opinions adopted require subsequent revision. 

 Sabine,* 5 by his own observations, which were made from 

 1818 to 1822 in very different zones of latitude, and by the 

 able arrangement and comparison of the numerous oscillation- 

 experiments with the vertical and horizontal needles, which 

 of late years have gradually become more general, has shown 

 that the intensity and inclination are very variously modi- 

 fied ; that the minimum of the terrestrial force at many 

 points lies far from the magnetic equator ; and that in the 

 most northern parts of Canada and in the Arctic regions 

 around Hudson's Bay from 52 20' lat. to the magnetic pole 

 in 70 lat. and from about 92 to 93 W. long, the intensity, 

 instead of increasing, diminishes. In the Canadian focus of 

 greatest intensity, in the northern hemisphere, found by 

 Lefroy, the dip of the needle in 1845 was only 73 7 7 and 



95 Fifth Report of the British Association, p. 72 ; Seventh Report, 

 pp. 64 68. Contributions to Terrestrial Magnetism No .vii iu the 

 Phil. Transact, for 1846, pt. iii, p. 254. 



