in different Latitudes. &51 



cate, and which is the increase of the magnetic force pro- 

 ceeding from the equator to the poles. 



To follow these results with more facility it will be proper 

 to set out from a fixed term, and it appears natural to make 

 choice ior that purpose of the points where the inclination 

 of the magnetic needle is null, because they seem to indi- 

 cate the places where tlie opposite action of the two terres- 

 trial hemispheres is equal. The series of these points forms 

 on the surface of the earth a curved line which difters very 

 sensibly from the terrestrial equator, from which it deviates 

 to the south in the Atlantic Ocean and to the north in the 

 South Sea, Thig curve has been called the magnetic equU" 

 ior, from its analoa;v to the terrestrial equator, though it is 

 not yet known whether it forms exactly a great circle of the 

 earth. We shall examine this question hereafter ; at pre- 

 sent it will be sufficient to say, that M. Humboldt found 

 this equator in Peru about 7' 7963" {T' V) of south latitude, 

 which places it, for that part of the earth, nearly in the 

 spot where Wilke and Lemonnier had iixed it. 



The places situated to the north of that point may be di- 

 vided into four zones; the three first of which, being nearer 

 the equator, are about 4-5° (4°) of breadth in latitude; 

 while the latter, more extensive and more variable, is 16** 

 (14"). So that the system of these zones extends in Ame- 

 rica from the magnetic equator to '25-553G'^ (23") of north 

 latitude, and comprehends in longitude an interval of about 

 56" (50"). 



The first zone extends from 7* ZOGS'^ (7" l") of south latitude 

 to 3*22° (2" 54'). The mean number of the oscillations of 

 the needle in the magnetic meridian in lO' of time is there, 

 21J'9: no ohservalioQ gives less than 211, or more than 

 214. From M. Humboldt's observations one might form 

 a similar zone on the south side of the magnetic equator, 

 which would give the same results. 



The second'zone extends from 2-4630" (2° 13') of south 

 latitude to 3'6l° (3" 13') of north btitude. The mean term 

 of the oscillations is there, 217'y: they are never below 

 220, or above 226. 



The fourth zone, broader than the other two, extends 

 from 10-2778" (9^ 15') to 25-7037° (23=' 6') of north lati- 

 tude. Its mean term is 23 7 : it never presents any ob- 

 servation below 239, or above 240. 



We are unacquainted, in regard to this pnrt of the earth, 

 with the intensity of the magnetic torcc beyond the latitude, 

 of 26' (23'') north; and on the other hand, in Europe, 

 where we have observations made in high latitudes, we 



ha\» 



