NOTES. liii 



Alexander VI. divided the western hemisphere between Spain and Portugal, 

 was not drawn through the westernmost of the group of the Azores, because 

 Columbus was desirous of converting a physical into a political division. He 

 attached great importance to the zone (raya), " in which the compass ceases 

 to shew any variation, where the air and the sea assume a new character, where 

 the sea is covered with weeds, and cool breezes begin to blow, and where the 

 form of the Earth is no longer the same." The last supposition was ail in- 

 ference from erroneous observations of the Pole star. 



C 52 ) p. 171. It is a question of the highest interest towards the solution 

 of the problem of the physical causes of terrestrial magnetism, whether the 

 two remarkable systems of closed isogonic curves will continue to move forward 

 for centuries to come, still preserving this closed form, or whether they will 

 open out and lose their peculiar character. In the Asiatic system, the decima- 

 tion increases from without inwards ; in the Pacific system, the contrary is the 

 case ; no line of no declination, indeed, no line below 2 east, is now known in 

 the Pacific to the east of Kamschatka, (Erman, in Poggend. Annalen, Bd. xxi. 

 S. 129): but it appears that, in 1616, on Easter-day, Cornelius Schouten 

 found no declination a little to the S.E. of Nukahiva, in 15 S. lat. and 132 

 W. long, (from Paris), and, therefore, in the middle of the present closed 

 system (Hansteen, Magnetismus der Erde, 1819, S. 28). In all these con- 

 siderations it must not be forgotten, that we can only trace the lines of magnetic 

 direction and force in their progressive displacements by observations which 

 are confined to their intersection with the surface of the Earth. 



( 153 ) p. 172. Arago, Annuaire, 1836, p. 284; and 1840, pp. 330338. 



( 154 ) p. 172. Gauss, Allgemeine Theorie der Erdmagnetismus, 31. 



( 155 ) p. 172. Duperrey, de la configuration de 1'e'quateur magnetique, in 

 the Annales de Chimie, T. xlv. pp. 371 and 379 ; also Morlet, in the Memoires 

 presentes par divers savans a 1'Acad. Roy. des Sciences, T. iii. p. 132. 



( 156 ) p. 173. See, in Sabine's Contributions to Terrestrial Magnetism, 

 1840, p. 139, the remarkable map of the isoclinal lines in the Atlantic Ocean 

 for 1825 and 1837. 



( 157 ) p. 174. Humboldt, iiber die seculare Veranderung der magnetischen, 

 Inclination, in Poggeud. Annalen, Bd. xv. S. 322. 



(!) p. 174. Gauss, Resultate der Beob. des magn. Vereins im Jahr 1838, 

 21 ; Sabine, Report on the Variations of the Magnetic Intensity, p. 63. 



[The progress of investigation, since the first volume of " Cosmos" was 

 written, has continued to confirm the opinion expressed by M. de Humboldt in 

 tlic text (p. 174), that in respect to the theory of terrestrial magnetism, the 



