MAGNETISM. 183 



decomposing substances, or a light-engendering magnetic 

 apparatus ! In such a chasm lie buried thousands of years 

 that compose the history of the intellectual development of 

 mankind ! 



The incessant change or oscillatory motion which we dis- 

 cover in all magnetic phenomena, whether in those of the 

 inclination, declination arid intensity of these forces, according 

 to the hours of the day and the night, and the seasons and 

 the course of the whole year leads us to conjecture the ex- 

 istence of very various and partial systems of electric currents 

 en the surface of the Earth. Are these currents, as in Seebeck's 

 experiments, thermo-magnetic, and excited directly from un- 

 equal distribution of heat ? Or, should we not rather regard 

 them as induced by the position of the Sun and by solar heat ?* 

 Have the rotation of the planets, and the different degrees 

 of velocity which the individual zones acquire according to 

 their respective distances from the equator, any influence on 

 the distribution of magnetism ? Must we seek the seat of 

 these currents, that is to say, of the disturbed electricity, in 

 the atmosphere, in the regions of planetary space, or in the 

 polarity of the Sun and Moon ? Galileo, in his celebrated 

 Dialogo, was inclined to ascribe the parallel direction of the 

 axis of the Earth to a magnetic point of attraction seated 

 in universal space. 



If we represent to ourselves the interior of the Earth as fused 

 and undergoing an enormous pressure, and at a degree of 

 temperature the amount of which we are unable to assign, we 

 must renounce all idea of a magnetic nucleus of the Earth. 

 All magnetism is certainly not lost until we arrive at a white 

 heat,| and it is manifested when iron is at a dark red heat j how- 



" The phenomena of periodical variations depend manifestly on the 

 action of solar heat, operating probably through the medium of thermo- 

 electric currents induced on the Earth's surface. Beyond this rude guess, 

 however, nothing is as yet known of their physical cause. It is even 

 still a matter of speculation, -whether the solar influence be a principal 

 or only a subordinate cause, in the phenomena of terrestrial magnetism." 

 (Observations to be made* in the Antarctic Expedition, 1840, p. 35.) 



t Barlow, in the Pliilos. Tram., for 1822, pt. i., p. 117; Sir David 

 Brewster, Treatise on Magnetism, p. 129. Long before the times of 

 Gilbert and Hooke it was taught in the Chinese work Oiu-thsa-tsou that 

 heat diminished the directive force of the magnetic needle. (Klaproth, 

 Lettre d M. A. de- Humboldt, sur I' Invention de la Bomsole, p. 96.) 



