INFLUENCE OF THE SUN AND MOON. 417 



from which is deduced 



, 



+ ~~ 



dr r n \rj a r4 \a 



For a point on the surface we have simply 



(8) dV n+i n 



The vertical component 



dr 



has for its general term 

 (9) Z^ 



This equation, combined with the preceding one (8), gives 



and we may thus easily separate the effect due to internal masses 

 from those produced by external masses. 



The calculations of Gauss having shewn that the observations 

 are satisfied by means of the single coefficients A, it follows that 

 the coefficients B are virtually null ; hence no sensible part of the 

 terrestrial action is due to external magnetic masses. 



441. INFLUENCE OF THE SUN AND MOON. There are, how- 

 ever, certain periodical variations in the elements of terrestrial 

 magnetism, which appear to be connected with the apparent motions 

 of the Sun and Moon, or at any rate to depend on certain accessory 

 phenomena such as the spots of the Sun. The influence of these 

 bodies can scarcely be doubted ; everything leads, however, to the 

 belief that they do not act directly as magnetic bodies, but that their 

 influence is indirect, and only modifies the magnetic condition of the 

 terrestrial globe. 



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