254 Variations of the Tenesfrial Magnetism 



geueral : they depend merely on the situation of the places 

 in regard to the magnetic equator, and belong to a general 

 phtenomenon, which is the increase of the intensity of the 

 magnetic forces in proportion as we remove from the cqua- 

 ttjr: the other kind of variations, which are much smaller 

 andaltoa:ether irregular, seem to depend entirely on local cir- 

 cumstances, andmodifyeithermoreor less the general results* 



If we consider terrestrial magnetism as the effect of an 

 attractive force inherent in all the material particles of the 

 globe, or only in some of these particles, which we are far 

 from determining, the general law will be, the total result 

 of the system of attraction of all the particles, and the small 

 anomalies will be produced by the particular attractions of 

 the partial systems of the magnetic moleculae diffused irre- 

 gulaily around each point; attractions rendered more sen- 

 sible by the diminution of distance. 



It now remains to consider the inclination of the mag-» 

 nelic needle in regard to the horizontal plane. It has long 

 been known that this inclination is not every where the 

 same: in the northern hemisphere the needle inclines to- 

 wards the north; in the southern towards the south; the places 

 where it becomes horizontal form the magnetic equator; and 

 those where the inclination is equal, but not null, form on 

 each side of that equator curved lines, to which the name of 

 magnetic parallels has been given from their analogy to 

 the terrestrial parallels. One may see in several works, and 

 particularly in that of Lemonnier, entitled Lois dii Mag- 

 iietism, the figure of these parallels and their disposition on 

 the face of the earth. 



It evidently rcsuks from this disposition that the inclina- 

 tion increases in proportion as we recede from the magnetic 

 equator; but the law which it follows in its increase has 

 not yet, as far as appears to us, been given. To ascertain 

 this law, however, would be of great utility; for the in- 

 clination seems to be the most constant of all the magnetic 

 phaenomena, and it exhibits much fewer anon)Hlies than the. 

 intensity. Besides, if any rule, well confirmed, could be dis- 

 covered on this subject, it might be employed with advan- 

 tage at sea to determine the latitude when the v/eather does 

 not admit an observation of the sun ; which is the case in 

 various places during the greater part of the year. We 

 have some reason to expect this application when wcr 

 see the delicalcv of that indication in the observations of 

 M. Humboldt, where wc find 0-65'' (35' 6") of difference 

 between two towns so near each other as Nismes and Mont- 

 peUier. These motives have induced us to study with great 



interest 



