Observations in Siberia* 7 



intensity ; to return to Krasnejark, and to cross, in a route 

 from thence to the Caspian sea, the curves 278, 287, and 297, 

 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 

 Kamtchatka, in which route he expects again to cross the same 

 curves, 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 ascer- 

 tained the reappearance on the Asiatic side, those of lesser 

 intensity passing altogether to the south of his present journey. 

 1 shall however briefly notice the remainder, in order to com- 

 plete the sketch of the isodynamic curves in the northern 

 hemisphere, as far as the observations will warrant. The next 

 curve, in which the needle would make n vibrations in 308 

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

 near the cities of Mexico and Carthagena ; by myself in 1822, 

 near TenerifFe ; and again by M. Humboldt at Madrid and in 

 the south of France. The next, in which the needle would 

 require 321 seconds for n vibrations, was observed both by M. 

 Humboldt and myself on the South American shore of the 

 Atlantic, near the 10th degree of north latitude ; and by my- 

 self was ascertained to pass to the north of Port Praya in the 

 Cape Verd islands. The next, in which the needle would make 

 n vibrations in 335 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 shewn by 

 my observations at Bathurst, where the intensity was greater, 

 and at Sierra Leone where it was less. The next, where the 

 needle would require 351 seconds for n vibrations, was observed 

 by M. Humboldt at Tompenda in Peru, on the western side of 

 South America; at Maranham on the eastern side by myself; 

 and on the African side of the Atlantic it enters the continent 

 of Africa, south of Sierra Leone. Finally, the curve of least 

 intensity which appears in this quarter of the northern hemi- 

 sphere, is that in which the needle would require 370 seconds 

 for n vibrations : in its progress from the southern hemisphere, 

 (where it was observed by myself at Bahia and Ascension), it 



