90 COSMOS. 



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,* 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 oscilla- 

 tion-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 

 modified ; that tlje 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 intensi- 

 ty, 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 

 in both hemispheres we find the maxima of the terrestrial 

 force coinciding with a comparatively small dip.| 



However admirable and abundant are the observations of 

 intensity which we owe to the expeditions of Sir James Ross, 

 Moore, and Clerk, in the Antarctic polar seas, there is still 

 much doubt in reference to the position of the stronger and 

 weaker focus in the southern hemisphere. The first of these 

 navigators has frequently crossed the isodynamic curves of 

 greatest intensity, and, from a careful consideration of his 

 observations, Sabine has been led to refer one of the foci to 

 64 S. lat. and 137 30' E. long. Ross himself, in the ac- 

 count of his great voyage, J conjectures that the focus lies in 



* F(fth Report of the British Association, p. 72; Seventh Report, p. 

 64-68. Contributions to Terrestrial Magnetism, No. vii., in the Phil. 

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



t Sabine, in the Seventh Report of the J3rit. Assoc., p. 77. 



j Sir James Ross, Voyage in the Southern and Antarctic Regions, vol. 

 i., p. 322. This great navigator, in sailing between Kergnelen's Land 

 and Van Diemen's Land, twice crossed the curve of greatest intensity, 

 first in 46 44' S. lat. 128 28' E. long., where the intensity increased 

 to 2-034, and again diminished, further east, near Hobnrton, to 1-824 

 ( Toy., vol. i., p. 103-104) ; then again, a year later, from January 

 1st to April 3d, 1841, during which time it would appear, from the 

 log of the Erebus, that they had gone from 77 47' S. lat. 175 41' 

 E. long, to 51 16' S. lat. 136 50' E. long., where the intensities were 

 found to be uninterruptedly more than 2-00, and even as much as 2-07 

 (Phil. Transact, for 1843, pt. ii., p. 211-215). Sabiue's result for the 

 one focus of the southern hemisphere (64 S. lat. 137 30' E. long.), 

 which I have already given ih the text, was deduced from observations 

 made by Sir James* Ross between the 19th and 27th of March, 1841 



