MAGNETIC PARALLELS. 407 



CHAPTER VI. 

 MAGNETIC CONDITION OF THE GLOBE. 



433. GAUSS' METHOD. The representation of terrestrial mag- 

 netism by the hypothesis of a central magnet, or by equivalent 

 hypotheses, only constitutes a somewhat rough first approximation ; 

 the problem is really far less simple. Gauss treated it in a com- 

 pletely general manner, on the hypothesis that the effects observed 

 on the surface of the Earth are due solely to the action of magnetic 

 masses. 



Whatever may be the distribution of these masses, whether they 

 are in the inside or on the surface of the globe, the elementary 

 actions are exerted inversely as the square of the distance, and the 

 force at each point is still determined by a potential. The space 

 surrounding the earth forms the magnetic field of the system, and we 

 may suppose it divided into layers by equipotential surfaces, corre- 

 sponding to equidistant values of the potential. The surface, which 

 corresponds to a given value V, may be formed of one or more 

 sheets ; but we know that two surfaces of different potentials do not 

 intersect, and that the force perpendicular at each point is inversely 

 as the distance of two consecutive surfaces. 



434. MAGNETIC PARALLELS. A certain number of these sur- 

 faces cut the terrestrial globe : magnetic parallels are the lines of 

 intersection corresponding with the surface of the Earth ; these lines 

 are level lines. As they belong both to the surface of the Earth, 

 which we suppose is spherical, and to the equipotential surface, they 

 are perpendicular at each point to the vertical and to the magnetic 

 force; they are therefore perpendicular to the magnetic meridian 

 passing through these two lines, and therefore to the intersection 

 of this meridian with the surface of the Earth that is, to the mag- 

 netic meridian. The magnetic parallels form, therefore, on the 

 surface of the terrestrial sphere, a system orthogonal to the mag- 

 netic meridians. 



