TERKESTRIAL MAGNETISM. 175 



cepting a small part of Russia, has now a western declination, 

 whilst at the close of the seventeenth century the needle first 

 Dointed due north, in London in 1657, and in Paris in 1669, 

 there being thus a difference of twelve years, notwithstanding 

 the small distance between these two places. In eastern 

 Russia to the east of the mouth of the Volga, of Saratow, 

 Nischni-Nowgorod and Archangel, the easterly declination of 

 Asia is advancing towards us. Two admirable observers, 

 Hansteeii and Adolphus Erman, have made us acquainted 

 with the remarkable double curvature of the lines of decli- 

 nation in the vast region of Northern Asia : these being con- 

 cave towards the pole between Obdorsk, on the Oby, and 

 Turuchansk, and convex between the Lake of Baikal and the 

 Gulf of Ochotsk. In this portion of the Earth, in northern 

 Asia, between the mountains of Werchojansk, Jakutsk, and 

 the northern Korea, the isogonic lines form a remarkable 

 closed system. This oval configuration* recurs regularly, 



for the 13th of September, 1492, the 21st of May, 1496, and the 16th 

 of August, 149S. The Atlantic line of no decimation at that period 

 ran from north-east to south-west. It then touched the South American 

 continent a little east of Cape Codera, while it is now observed to reach 

 that continent on the northern coast of the Brazils. (Humboldt, Examen 

 critique de I'Hist. de la Geogr., t. iii. pp. 44-48.) Prom Gilbert's 

 Physiologia Nova de Magnete, we see plainly (and the fact is veiy 

 remarkable) that in 1600 th<. declination was still null in the region of 

 the Azores, just as it had been in tht time of Columbus (lib. 4, cap. 1.) 

 I believe that in my Examen critique (t. iii. p. 54) I have proved from 

 documents, that the celebrated line of demarcation by which Pope 

 Alexander VI. divided the "Western hemisphere between Portugal and 

 Spain, was not drawn through the most western point of the Azores, 

 because Columbus wished to convert a physical into a political division. 

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

 shows no variation, where air and ocean, the latter covered with pastures 

 of sea-weed, exhibit a peculiar constitution, where cooling winds hegin 

 to blow, and where [as erroneous observations of the polar star led him to 

 imagine] the form (sphericity) of the Earth is no longer the same." 



* To determine whether the two oval systems of isogonic lines, so 

 singularly included each within itself, will continue to advance for cen- 

 turies in the same inclosed form, or will unfold and expand themselves, 

 is a question of the highest interest in the problem of the physical causes 

 of terrestrial magnetism. In the Eastern Asiatic nodes the declination 

 increases from without inwards, while in the node or oval system of the 

 South Sea the opposite holds good; in fact at the present time, in 

 the whole South Sea to the east of the meridian of Kamtschatka, 

 there is no line where the decimation is null, or indeed in which it is 



