and Thermo- Magnetism of the Earth. 313 



case to contain water, the sides being surrounded with a bad 

 conductor of heat, except a space left to be covered with 

 double glass, for the purpose of observation ; for it appears 

 from my experiments that the needle is not much aiFected by 

 a change of temperature at the sides of the box only. 



It has till within a very recent period been generally as- 

 sumed that the earth's magnetism is owing to a central mag- 

 net, notwithstanding the incongruity of many facts with such 

 an hypothesis. Indeed, if we admit the existence of intense 

 heat in the interior of the globe, we have every reason to be- 

 lieve that magnetism cannot exist there; since it appears, that 

 neither the loadstone nor steel can retain it at a high tem- 

 perature, and that iron at a white heat loses its power of at- 

 tracting the needle. 



The discoveries of Oersted and Seebeckhave, however, laid 

 the foundation of juster views of this interesting subject, and 

 many difficulties vanish when the phaenomena of the earth's 

 magnetism are referred to the circulation of electrical currents 

 around it. This hypothesis, which was first suggested by 

 Ampere, appeared to me to derive strong confirmation from 

 the stratification of rocks, the arrangement of metallic and 

 other veins, the high temperature which in a greater or less 

 degree prevails under the surface of the earth, and its rota- 

 tion on its axis, possessing as they seemed to do, many ana- 

 logies to electro-magnetic, and thermo-electric combinations. 

 I was consequently led to suspect the existence of free electri- 

 city in metalliferous veins, and was not disappointed*. 



If we take a glance at the map of the world, we perceive 

 that it consists of two grand divisions of land, and two of 

 water, alternating with each other, from east to west. This 

 curious arrangement seems to bear on the point in question, 

 as well as the difference of temperature found generally to 

 exist between the eastern and western sides of great conti- 

 nents; the lines of minimum temperature may possibly coin- 

 cide with those of no variation ; at least this point seems to 

 deserve investigation when opportunities^ occur. 



The direction of the electrical currents under the earth's 

 surface may be greatly diversified ; this may be inferred from 

 my experiments on the electricity of metallic veins f. But the 

 facts I have referred to, especially the rotation of our planet 

 from west to east, and the solar rays acting in a contrary di- 

 rection, would induce us to suppose that the currents, taken col- 



• In some lead-mines in Flintbhire, where the temperature is low, I 

 could not detect any free electricity. Was this fact owing to their being 

 situated in horizontal strata? 



t See Fhil. Trans. 1830, p. 400, &:c. 



Third Scries. Vol. 1. No. !■. Oct. 1832. 2 S 



