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. 
Erman, 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 myself, 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 found by myself at the island of St. Thomas, in the 
