SECT. XXXV. THE EARTH NOT A REAL MAGNET. 331 



netic induction, on a magnetized needle, would be, if 

 insulated from the influence 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 center 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 

 induction, and not a real magnet. Mr. Barlow has ren- 

 dered 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 influence of the earth's magnetism, 

 exhibited all the phenomena of the dipping and varia- 

 tion needles, according to its positions with regard to 

 the wooden globe. As there can be no doubt that the 

 same phenomena would be exhibited by 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 

 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 disturbance in the direction of the magnetic lines, in 

 the same manner as it influences the parallelism of the 

 isothermal lines. That such currents do exist in metal- 

 liferous veins appears from the experiments of Mr. Fox 

 in the Cornish mines. Even since the last edition of 

 this book was published, Mr. Fox has obtained additional 

 proof of the activity of electro-magnetism under the 

 earth's surface. He has shown that not only the nature 

 of the metalliferous deposits must have been determined 



