150 Magnetism of the Earth. 



midway between Stockholm and Tornea. He has since traced its 

 prolongation through St. Petersburg and Moscow. 



It is M. Hansteen's intention to commence the present summer by 

 descending the Jenisei to Touroukansk, under the polar circle, in or- 

 der to extend the tracing of the curve No. 1 : returning to Krasno- 

 jarsk, to cross, in a route from thence to the Caspian Sea, the curves 

 2, 3 and 4, in their further prolongation to the south east : whilst Dr. 

 Errnan, who quits him at Irkutsk, and is furnished with the necessary 

 instruments, will proceed by Jakutsk and Ochotsk to Kamschatka, 

 in which route he expects again to cross the same curves 2, 3 and 4, 

 after they have passed their southern Asiatic limit, and resumed for 

 a second time a north easterly direction. 



These are all the curves of which M. Hansteen has ascertained 

 the reappearance on the Asiatic side, those of lesser intensity pass- 

 ing altogether to the south of his present journey. I shall, however, 

 briefly notice the remainder, in order to complete the sketch of the 

 isodynamic curves in the northern hemisphere. No. 5, in which the 

 needle would make one hundred vibrations in three hundred and eight 

 seconds, was observed by M. Humboldt in 1800 — 1805, to pass near 

 the cities of Mexico and Carthagena; by myself in 1822 between 

 Teneriffe and Madeira ; and again by M. Humboldt at Madrid and 

 in the south of France. No. 6, in which the needle would require 

 three hundred and twenty one seconds for its one hundred vibrations, 

 was observed both by M. Humboldt and mj^self, on the South Amer- 

 ican shore of the Atlantic, near the 10th degree of north latitude; 

 and by my observations at Port Praya, was ascertained to pass to the 

 north of the Cape Verd Islands. No. 7, in which the needle would 

 make one hundred vibrations in three hundred and thirty five seconds, 

 was frequently observed by M. Humboldt in the interior and on the 

 western side of Columbia ; after crossing the Atlantic, it enters the 

 continent of Africa somewhat to the south of the Gambia River, as is 

 shown by my observations at Bathurst where the intensity was great- 

 er, and at Sierra Leone where it was less. No. 8, where the needle 

 would require three hundred and fifty one seconds, was observed by 

 M. Humbolt at Tompenda in Peru on the western side of South 

 America ; at Marenham on the eastern side, by myself; and on the 

 African side of the Atlantic, enters the continent of Africa somewhat 

 south of Sierra Leone. Finally, No. 9, in which the needle would 

 require three hundred and seventy seconds for its one hundred vi- 

 brations, was foinid by myself at the island of St. Thomas, in the 



