C. GENERAL PHYSICS. 155 



This is regarded as a companion to the "Magnetic Survey 

 of the South Polar Regions," published five years ago. The 

 present work differs from its companion in having been ex- 

 ecuted by the authority of a single nation and at the national 

 expense, and completed in the interval between the years 

 1 840-45, and thus requiring no corrections for secular changes, 

 while these corrections are an important feature in the earlier 

 communication, which comprehends the labors of many na- 

 tions between the years 1820-65. The earliest conclusion of 

 a systematic character regarding the phenomena of terres- 

 trial magnetism, and which has been borne out by our recent 

 knowledge, is that of Halley, in 1663, who concluded that 

 the globe of the earth may be regarded as one great magnet, 

 having four magnetic poles or points of attraction, two of 

 them near each pole of the earth, and two in those parts 

 which do not lie near either of those poles, the needle being 

 governed by the strongest pole, or being always predominant 

 over the weaker. The next step in our knowledge was mark- 

 ed by the publication of Hansteen's work, " Der Magnetismus 

 der Erde" which brought together systematically all known 

 observations of the magnetic declination, and formed from 

 them maps of the phenomena corresponding to successive 

 epochs. The present contribution is a continuation in the 

 same direction; but the amount of new material which has 

 accumulated since the publication of Hansteen's work, in 

 1819, shows the increasing interest in this subject, and may 

 be regarded as constituting an era in the history of its pro- 

 gressive advance. The earliest authoritative knowledge we 

 possess of the magnetic state of North America was contain- 

 ed in the same communication from Halley above referred to, 

 in 1663, at which time the North American pole was stated 

 by him to be in the meridian corresponding with the middle 

 of California, and about 15 from the north pole of the globe. 

 The latest determination that of Sir Edward Sabine, as 

 given in his present communication places the maximum 

 of magnetic inclination in latitude 70 north, and longitude 

 263 east, while the maximum of magnetic force is in latitude 

 53 north, and longitude 268 east, both localities being to 

 the east and a little to the south of the geographical position 

 which Halley assigned to the magnetic pole in 1663. The 

 elaborate maps of the northern regions accompanying the 



