72, TERRESTRIAL MAGNETISM. 



and W. longitude 78 46', and where the intensity is taken 

 as=rOOO. It was to this point (Humboldt, Becueil 

 d'Observ. astr., T. ii. p. 382385 ; and Voyage aux 

 Eegions equinox. T. iii. p. 622) that for forty years the 

 reductions in all tables of the force were referred as a basis 

 (Gay-Lussac, in the Mem. de la Soc. d'Arcueil, T. i. 1807, 

 p. 21 ; Hansteen liber den Magnetismus den Erde, 1819, 

 S. 71 ; Sabine, in the Report of the British Association at 

 Liverpool, p. 43 58.) It has since been justly objected, 

 that the point so taken as unity does not afford an appro- 

 priate general standard, since the line of no inclination ( 71 ) 

 does not correspond with the points of weakest intensity 

 in many meridians, and that no point on the earth's surface 

 can be taken as a permanent unity, on account of secular 

 change. (Sabiiie, in the Phil. Trans, for 1846, Part iii. 

 p. 254 ; and in the Manual of Scientific Inquiry for the use 

 of the British Navy, 1849, p. 17.) 



1828 1829. Hansteen and Due's expedition to Siberia : 

 magnetic observations in European Eussia and Eastern 

 Siberia, as far as Irkutzk. 



1828 1830. Adolph Erman's journey through Northern 

 Asia, and voyages in the Pacific and Atlantic Oceans in 

 the Eussian frigate Krotkoi. The identity of the instru- 

 ments used, the employment of the same or similar methods 

 throughout, the exactness of the astronomical determinations 

 of geographical position, and the whole of the observations 

 being made by the same thoroughly informed and practised 

 observer, returning to the same point after having gone 

 round the globe, are all circumstances which combine in 

 assigning a high value to this enterprise, executed at 

 private expense. (See the General Map of the Declination, 



