378 THE EARTH NOT A REAL MAGNET, [SECT. xxxv. 



this subject with much skill and success. He first proved 

 that the magnetic power of an iron sphere resides in its 

 surface ; he then inquired what the superficial action of an 

 iron sphere, in a state of transient magnetic induction, on 

 a magnetized needle, would be, if insulated from the in- 

 fluence of terrestrial magnetism. The results obtained, 

 corroborated by the profound analysis of M. Poisson, on the 

 hypothesis of the two poles being indefinitely near the 

 centre of the sphere, are identical with those obtained by 

 M, Biot for the earth, from M. de Humboldt's observations ; 

 whence it follows, that the laws of terrestrial magnetism 

 deduced from the formulae of M. Biot are inconsistent with 

 those which belong to a permanent magnet, but that they 

 are perfectly concordant with those belonging to a body in 

 a state of transient magnetic induction. The earth, there- 

 fore, is to be considered as only transiently magnetic by in- 

 duction, and not a real magnet. Mr. Barlow has rendered 

 this extremely probable by forming a wooden globe, with 

 grooves admitting of a copper wire being coiled round it 

 parallel to the equator from pole to pole. When a current 

 of electricity was sent through the wire, a magnetic needle 

 suspended above the globe, and neutralized from the in- 

 fluence of the earth's magnetism, exhibited all the phe- 

 nomena of the dipping and variation needles, according to 

 its positions with regard to the wooden globe. As there can 

 be no doubt that the same phenomena would be exhibited 

 jby currents of thermo, instead of Voltaic, electricity, if the 

 grooves of the wooden globe were filled by rings constituted 

 of two metals, or of one metal unequally heated, it seems 

 highly probable that the heat of the sun may be a great 

 agent in developing electric currents in or near the surface 

 of the earth, by its action upon the substances of which the 

 globe is composed, and, by changes in its intensity, may 

 occasion the diurnal variation of the compass, and the other 

 vicissitudes in terrestrial magnetism evinced by the dis- 

 turbance in the directions of the magnetic lines, in the 



