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XXXVII. 0?? the Variations of the Terrestrial Magnetism- 

 in different Latitudes. By Messieurs Humboldt and 

 BiOT. Read ly JM. Biot, in the Mathematical and 

 Physical Class of the French National Institute 2^tli 

 Primaire, An 13*. {\"!th December 1S04.) 



An inquiry into the laws of terrestrial magnetism is no 

 doubt one of the most important questions that philoso- 

 phers can propose. The observations already made on this 

 subjt'ct have discovered phasnomena so curious, that one 

 cannot help endeavouring to solve the difficulties they pre- 

 sent ; but notwithstanding the efforts hitherto emplovcd, it 

 irjust be confessed that we are absolutely unacquainted with 

 the causes of them. 



It was difficult to obtain on this point any precise know- 

 ledge at a time when the construction of the compass 

 was stiH imperfect; and so little time has elapsed since 

 the discoveries of M. Coulomb have taught us to render 

 them completely exact, it needs excite no astonishment that 

 so few facts in the observations of travellers have been 

 found worthy of confidence. 



The expedition which M. Humboldt has terminated has 

 procured for this part of philosophy a collertion no less 

 valuable than those v. ith which he has enriched the other 

 branches of human knowledge. Furnished with an excel-, 

 lent dippmg-needle, constructed by Le Noir on the princi- 

 ples of iierda, iVI. Humboldt has made more than three 

 bundled observations on the inclination of the magnet, anci 

 on the intensity of the magnetic force in that part of Ame- 

 rica which he traversed. By adding to these results those, 

 which he had already obtained in Europe before his de- 

 parture, we shall have for the first time a series of correct 

 facts on the variation of the magnetic forces in the northern 

 part of the globe, and in some points of its. southern 

 part. 



The fricndsliip which IM. Flumboldt has testified for me 

 since his r-tvirri having given me an opportunity of pomiT\u- 

 r-icating to him some experiments on this subject, which 

 I made this year in the Alps, he immediately ottered to 

 unite his to mine in a memoir. But if friendship and a 

 desire of making known new facts induced me to accept 

 this oifer of M. Humboldt, justice forbids me to take ad- 

 vantage of it to his prejudice ; and I must here declare, that 

 ■i. very small part of it belongs to me. 



* i^amt\it Journal dc Phyriquc, Frimaire, An 13. 



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