Magnetism of the Earth. 149 



It may be remarked of the Siberian curve, that the space which it 

 encloses is considerably less than its parallel in America ; a circum- 

 stance consistent with the supposition already noticed, that the maxi- 

 mum intensity in Siberia is inferior in attractive force to the maximum 

 in Hudson's Bay : consequently curves of equal intensity are encoun- 

 tered at a less distance from the point of maximum in Siberia than in 

 America. 



The second curve on the American side connects those places 

 where the needle, introduced for illustration, would perform its one 

 hundred vibrations in two hundred and seventy eight seconds. The 

 points which have determined it are, Melville Island in the north west; 

 several stations on the west side of Greenland, from lat. 76° to lat. 

 66°, in the north east ; and finally a greater intensity observed at 

 New York and a lesser at the Havanna ; whence it is concluded that 

 this curve intersects the sea board of the United States at an inter- 

 mediate point between those cities. A corresponding intensity has 

 been traced by Dr. Erman of Berlin, (who accompanied M. Han- 

 steen to Siberia,) from the mouth of the River Oby, in lat. 68° and 

 long. 70° E. preserving nearly the direction of a meridian, to lat. 60°, 

 whence it bends gradually to the eastward, passes between Tobolsk 

 and Naryon, and has been observed at Kainsk by M. Hansteen, on 

 its way to its probable southern hmit on the Asiatic side, a few de- , 

 grees south of Lake Baikal. 



No. 3 is that in which the needle would perform one hundred vi- 

 brations in two hundred and eighty seven seconds. Its points of ob- 

 servation have been, north of the Havanna, east of the Pendulum Isl- 

 ands on the eastern side of Greenland in lat. 74° 5', between the 

 North Cape and Spitzbergen, by myself in 1823, and by M. Keilhau 

 in 1827. By M. Hansteen's observations it enters the continent of 

 Europe between Archangel and Nova Zembla, and was crossed by 

 him, on the route from Moscow to Tobolsk, in 56° and 57° east lon- 

 gitude, and 57° and 58° latitude. 



The curve marked No. 4 is that in which the needle would make 

 one hundred vibrations in two hundred- and ninety seven seconds. 

 Its tracing by observation commences on the American side, with the 

 islands of Caymen and Jamaica. Crossing the Atlantic, it passes 

 through the northern parts of the British islands, and enters Norway 

 south of Bergen. It there became subject to M. Hansteen's observ- 

 ation, who has ascertained its northern limit, and where it begins to 

 bend to the southward, to be on llie shores of the gulf of Bothnia, 



