

230 



ELECTEICITY AND MAGNETISM CONSIDERED [Cii, XXXIIL 



Electricity and magnetism considered as sources of volcanic 

 heat. — The popular notion of a central fmid nucleus^ on which 

 a thin outer shell is floating, has diverted the speculations 



om 



some 



theory which might explain the continual 

 shifting of the points of the chief development of heat from 

 one part of the shell to another^ leaving large portions 

 previously in a state of fusion to cool and consolidate. 

 Soon after the first great discoveries of Oersted in electro- 



Ampere suggested that all the phenomena 



magnetic needle might be explained by supposing currents 

 of electricity to circulate constantly in the shell of the globe 



magnetic equator. 



This 



in directions parallel to the 

 theory has acquired additional consistency the farther we 

 have advanced in science; and according to the experiments of 

 Mr. Fox, on the electro-magnetic properties of metalliferous 



trace of electric currents seems to have been 



venis. some 



detected in the interior of the earth."^ 



Some i3hilosophers ascribe these currents to the chemical 

 action going on in the superficial parts of the globe to which 

 air and water have the readiest access; while others refer 

 them, in part at least, to thermo-electricity excited by the 

 solar rays on the surface of the earth during its rotation ; 



atmospl 



posed to the influence of the sun, and then cooled again in 



That this idea is not a mere speculation, is 



the night. 



proved by the correspondence of the diurnal variations of the 

 magnet Avith the apparent motion of the sun ; and by the 



b 



summer 



during the day than in the night. 



Allusion was made in the first volume (p. 303) to the recent 

 discovery of a connection between periodical changes in the 

 spots of the sun and variations in terrestrial magnetism. 



suggesting the idea that solar mafjnetism 



According to 



a powerful 

 Sir John 



influence on the earth's crust. 

 Herschel, the cycle of change, including the periods when 

 the spots are most abundant and large, and those when they 

 are least apparent, occupies rather more than 11 years, 



^ Phil. Trans. 1830, p. 399. 



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