TE.RRESTRIAL MAGNETISM. 171 



Azores. ( 151 ) At the present time the whole of Europe, 

 with the exception of a small part of Bussia, has west de- 

 clination. This only began to be the case late in the 

 17th century, the needle having first pointed due north in 

 London in 1657, and in Paris in 1669, an interval of 

 twelve years, notwithstanding the small distance which 

 divides these two capitals. In eastern Russia, east of 

 the mouth of the Volga, of Saratow, Nishni-Nowgorod, 

 and Archangel, the easterly declination of Asia is advancing 

 towards us. In the wide extent of Northern Asia, two 

 excellent observers, Hansteen and Adolph Erman, have 

 traced the extraordinary double curvature of the declination 

 lines, which are concave towards the pole between Obdorsk 

 on the Obi and Turucjiansk, and convex between Lake 

 Baikal and the Gulf of Ochotsk. In this last-named por- 

 tion of the earth, in North-eastern Asia, between the 

 Werchoiansk Mountains, lakoutsk, and Northern Corea, 

 the isogonic lines form a remarkable closed system. This 

 oval form ( 152 ) is repeated with still greater regularity, and 

 on a larger scale, in the Pacific Ocean, nearly in the meridian 

 of Pitcairn Island and the group of the Marquesas, between 

 20 N. and 45 S. latitude. We might be inclined to 

 regard so singular a configuration as the effect of local 

 peculiarity in those parts of the earth ; but if these 

 apparently isolated systems are found to change their place 

 progressively in the course of centuries, we must conclude 

 that these phenomena, like all great natural facts, appertain 

 to a general system, and have a general cause. 



The horary variations of the decimation are apparently 

 governed by the sun whilst that body is above the horizon at 

 any spot; they also decrease in angular value with the decrease 



