On the Variaiioiu of the Terrestrial MagneSisin. 249 



To place in order the facts and consequences which may 

 he deduced from them, it is necessary to consider the ac- 

 tion oi terrestrial magnetism under diftereat points of view, 

 corresponding to tlie different classes of the phrenointna 

 which it produces. 



If we consider it first in general, we find (hat it acts on 

 the whole surface of the globe, and that it extends beyond 

 it. This last fact, which was doubted, has been lately- 

 proved by one of us^ and particularly by our friend M. Gav- 

 Lussac, duriiig his two aerostatic voyages. And if these 

 observations, made witli all the care possible, have not 

 shown the least sensible diminution in tiie intensity of the 

 magnetic force, at the greatest height to which man can 

 attain, we have a right to conclude that this force e.vieuds 

 to an indefinite distance from the earth, where it decreases, 

 perhaps, in a very rapid manner, but which at present is 

 unknown to us. 



If we now consider magnetism at the surface even of the 

 earth, we shall find three grand classes of phcenomena, 

 which it is necessary to study separately, in order tJ have a 

 complete knowledge of its mode of action. 1 hese phaeno- 

 mena are; the declination of tlie magnetic needle, its incli- 

 nation, and the intensity of the magnetic force, considered 

 tithcr comparatively in ditlerent places or in themselves, 

 paying attention to the variations which they exj)erience. 

 It is thus tiiat, after having discovered the action of gravity 

 ,18 a central force, its varia'ion, resulting from the figure of 

 the earth, \yas afterwards ascertained in ditlerent lati- 

 tudes. 



The declination of the maonctic needle appears to be 

 that pha;nonicnon Vvhich hitherto has more particularly 

 fixed the attention of philosophers, on account, no doubt, 

 ot the assistance which they hoped to derive from it in de- 

 termining the longitude; but wlicn it ^as known that the 

 declination change.; in the same place, in the course of 

 time, when its diurnal variations were remarked, and its 

 irregular traversing, occasioned by different meteors ; in a 

 word, the dithcul'v of observing it at sea, within one de- 

 gree nearly, it was necessary to abandon that hope, and to 

 consider the cause of these phaKnomcna as much u, ore com- 

 plex and abstruse than had been at first imagined. 



In regard to the intensity of the magnetic force in diffe- 

 rent parts of the earth, it has never vet been measured in a 

 comparative manner. The observations of M. Humboldt 

 on this subject have discovered a very remarkable pluruo- 

 fiicnon ; it is xhf: vari.ilion of the imoiTiilv in different jati- 

 1 tildes. 



