TEREESTRIAL MAGNETISM. 177 



The perpetual fluctuation observed in all the magnetic 

 phenomena, in the inclination, declination, and intensity of 

 the force, according to the hours of the day and even of the 

 night, the season of the year, and the lapse of years, leads to 

 the belief in the existence of very various and complicated 

 systems of electric currents in the crust of the Earth. ( 163 ) 

 Are these, as in Seebeck's experiments, simple thermo- 

 magnetic currents, the immediate effect of unequal dis- 

 tribution of heat, or currents induced by the calorific 

 action of the Sun ? Has the rotation of the Earth, and the 

 velocity of its different zones according to their distance from 

 the equator, any influence on the distribution of magnetism ? 

 Is the source of magnetic action to be sought in the atmo- 

 sphere, or in the interplanetary spaces, or in a polarity of 

 the Sun and Moon ? Galileo, in his celebrated " Dialogo," 

 ascribes the constant parallel direction of the Earth's axis to 

 a center of magnetic attraction existing in space. 



If we conceive the interior of the Earth to be molten, 

 subject to enormous pressure, and raised to a temperature 

 for which we possess no measure, we must renounce the 

 idea of a magnetic nucleus. Though at a white heat all 

 magnetism disappears, ( 164 ) it is still sensible in iron heated 

 to a dark-red glow; and whatever may be the modifica- 

 tions which, in these experiments, the molecukxr condition 

 and consequently the coercitive force undergo, there must 

 still remain a considerable thickness of terrestrial strata, in 

 which we might seek the seat of the magnetic currents. In 

 the old explanation of the horary variations of the declina- 

 tion, by the progressive warming of the Earth by the Sun 

 in his apparent course from east to west, the action would 

 indeed be limited to the extreme exterior surface ; for ther- 



