Electric Currents on the Earth, 53 



mena of terrestrial magnetism can be more readily accounted 

 for by the latter theory.* 



The hypothesis that currents of electricity circulate round 

 the earth in parallels of latitude, is grounded (chiefly, I be- 

 lieve) on the phenomena of thermo-electricity, from which it 

 is assumed, that as the earth is unequally heated by the sun, 

 there must be currents of electricity on its surface, the earth 

 being thus a grand thermo-electrical machine ; and as the 

 magnetic needle takes a position at right angles to the con- 

 ducting wires of a voltaic battery, that, therefore, these cur- 

 rents on the earth's surface must be at right angles to the 

 magnetic meridian. This theory has been adopted by many 

 of our most eminent philosophers ; but I trust I may not be 

 charged with presumption in pointing out some objections 

 which appear to me to tell against it. 



There has never, I believe, been any distinct proof, by ex- 

 periment^ of any currents on the earth crossing the magnetic 

 meridian. The experiments of Mr Fox shew, that the currents 

 he detected were generally from north to south, but it is pro- 

 bable the whole of them were owing to local causes. 



Again, the intensity of terrestrial magnetism is weakest at 

 or near the equator ; whereas, if it be caused by thermo-elec- 

 tricity, it would probably be most intense where the sun acts 

 with the greatest power ; and it is also reasonable to sup- 

 pose that there would be a great difference in the intensity 

 of terrestrial magnetism during the summer and winter 

 months in our own and higher latitudes. 



* Theories have been proposed from time to time, to account for the pheno- 

 mena of terrestrial magnetism on the hypothesis that they are caused, wholly 

 or in part, by some magnetic agency situated in the interior of the earth, or 

 that the earth itself acts as a magnet. 



It would be an interesting experiment to ascertain if there is any diifference 

 in the weight of unmagnetized iron in high and low latitudes, more than can 

 be accounted for by the form of the earth ; for if any of the phenomena of mag- 

 netism are caused by magnetic influence within the earth, it is probable there 

 would be, if such opinion be correct, an increase in the weight of iron corre- 

 sponding with the increase of the dip of the needle, or rather an appearance of 

 increase of weight from the addition of magnetic attraction to the specific gra- 

 vity of the iron. 



