REPORT ON THE MAGNETISM OF THE EARTH. 1 I.*? 



latter part of the morning, and the subsequent easterly motion, 

 by supposing that the heat of the sun acted upon the northern 

 parts of the earth as upon a magnet, by weakening their in- 

 fluence, but offered no explanation of the morning easterly mo- 

 tion of the needle. 



Oersted's discovery of the influence of the closed voltaic 

 circuit upon the magnetic needle, and the consequent discoveries 

 of Davy, Ampere and Arago, immediately led to the considera- 

 tion, whether all the phaenomena of terrestrial magnetism were 

 not due to electric currents ; and the discovery of Seebeck, that 

 electric currents are excited when metals having different 

 powers of conducting heat are in contact, — which discovery 

 with but few holds the rank to which it is eminently entitled, — 

 pointed to a probable source for the existence of such currents. 

 At the conclusion of a highly interesting paper on the develop- 

 ment of electro-magnetism by heat. Professor Gumming re- 

 marks that "magnetism, and that to a considerable extent, it 

 appears, is excited by the unequal distribution of heat amongst 

 metallic, and possibly amongst other bodies. Is it improbable 

 that the diurnal variation of the needle, which follows the 

 course of the sun, and therefore seems to depend upon heat, 

 may result from the metals, and other substances which com- 

 pose the surface of the earth, being unequally heated, and con- 

 sequently suffering a change in their magnetic influence ? " And 

 in the second part of a paper, detailing some thermo-magnetical 

 experiments, read before the Royal Society of Edinburgh, 

 Dr. Traill considers "that the disturbance of the equilibrium 

 of the temperature of our planet, by the continual action of the 

 sun's rays on its intertropical regions, and of the polar ices, 

 must convert the earth into a vast thermo-magnetic apparatus : " 

 and "that the disturbance of the equilibrium of temperature, 

 even in stony strata, may elicit some degree of magnetism*." 

 Previous to this, I had adopted the opinion that temperature, 

 if not the only cause, is the principal one of the daily variation f. 

 It did not, however, appear to me, that any of the experiments 

 hitherto made bore directly on the subject, since the metals 

 producing electric currents by their unequal conduction of heat 

 were only in contact at particular parts, and in no case had 

 such currents been excited by different metals having their 

 surfaces symmetrically united throughout. I in consequence 

 instituted a series of expei-iments with two metals so united, 

 and found that electric currents were still excited on the 



* Tr<fn.saclioni of the Ph'tlosophieal Soeiefy of Cambridge, xo\. ii. p. 64. 

 + Ph'ilosophk-nl Traihiacfions, 1823, p. 392. 



18,']rJ. I 



