92 COSMOS. 



what is cosmically of very great importance, historical points 

 of departure for those alterations in the force which will be 

 manifested in future years, probably through the dependence 

 of the earth upon the magnetic force of the sun, by which it 

 is influenced. 



In the northern hemisphere the stronger or Canadian 

 focus, in 52 19' N. lat. and 92 W. long., has been most 

 satisfactorily determined by Lefroy. This intensity is ex- 

 pressed in the relative scale by 1-878, the intensity of Lon- 

 don being 1-372, while in the absolute scale it would be ex- 

 pressed by 14-21.* E>en in New York, lat. 40 42 X , Sabine 

 found the magnetic force not much less (1-803). For the 

 weaker northern or Siberian focus, 70 lat., 120 E. long., 

 it was found by Erman to be 1*74 in the relative scale, and 

 by Hansteen 1-76 ; that is to say, about 13-3 in the absolute 

 scale. The Antarctic expedition of Sir James Ross has 

 shown us that the difference of the two foci in the southern 

 hemisphere is probably less than in the northern, but that 

 each of the two southern foci exceeds both the northern in 

 intensity. The intensity in the stronger southern focus, 64 

 lat., 137 30' E. long., is at least 2-06 in the relative or ar- 

 bitrary scale,f while in the absolute scale it is 15-60 ; in the 

 weaker southern focus, 60 lat, 129 W W. long., we find 

 also, according to Sir James Eoss, that it is 1'96 in the ar- 

 bitrary scale and 14-90 in the absolute scale. The greater or 

 lesser distance of the two foci from one another in the same 

 hemisphere lias been recognized as an important element of 

 their individual intensity, and of the entire distribution of 

 the magnetic force. " Even although the foci of the south- 

 ern hemisphere exhibit a strikingly greater intensity (name- 

 ly, 15-60 and 14-90 in the absolute scale) than the foci of 



* On the map of isodynamic lines for North America, which occurs 

 in Sabine's Contributions to Terrestrial Magnetism, No. vii., we find, by 

 mistake, the value 14*88 instead of 14-21, although the latter, which 

 is the true number; is given at page 252 of the text of this memoir. 



f I follow the value given in Sabine's Contributions, No. vii., p. 252, 

 namely, 15-GO. We find from the Magnetic Journal of the Erebus 

 (Phil Transact, for 1843, pt. ii., p. 169-172) that several individual 

 observations, taken on the ice on the 8th of February, 1841, in 77 47' 

 S. lat. and 172 42' W. long., yielded 2-124. The value of the intens- 

 ity 15-GO in the absolute scale would lead us to assume provisionally 

 that the intensity at Hobarton was 13*51 (Magn. an (^ Meteor ol. Observ. 

 made at Hobarton, vol. i., p. 75). This value has, however, lately 

 been slightly augmented (to 13-56) (vol. ii., xlvi.). In the Admiralty 

 Manual, p. 17, I find the southern focus of greatest intensity changed 

 to 15-8. 



