2 Prof. W. A. Norton on Terrestrial Magnetism. 



he was enabled to trace the magnetic equator, with peculiar ac- 

 curacy, through an extent of 247° of longitude. In his paper 

 on the magnetic equator, subsequently published, he announced 

 that he had discovered that " the points of this great circle, or 

 those where the magnetic intensity is a minimum, are also the 

 warmest points of each meridian," and thus that " the thermal 

 and magnetic equator are connected, as Sir David Brewster had 

 already proved to be the case with the thermal and magnetic 

 poles ;" also, " that in comparing the isothermal and isodynamic 

 lines, he had found a remarkable analogy in their curvatures and 

 particularly in the direction of their concavities, and convexities." 

 M. Kupffer, in certain memoirs read before the Russian Academy 

 about the year 1829, attempted to establish that terrestrial mag- 

 netism resides at the surface of the globe, and thence inferred the 

 existence of a connection between the magnetic and thermal phe- 

 nomena of the earth : conceiving that the intensity of the earth's 

 magnetism would vary directly or inversely as the temperature, 



netism. 



as it was of the nature of permanent 

 coniectures have been formed as to 



connection between the temperature and magnetism of the earth. 

 Dr. Traill has expressed the opinion 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 by the 

 polar ices, must convert the earth into a vast thermo-magnetic ap- 

 paratus." Christie has suggested that "difference of temperature 

 may be the primary cause of the polarity of the earth, though its 

 influences may be modified by other circumstances." CErsted 

 conceives that the sun, by producing evaporation, deoxydation, 

 &c, as well as by increasing the temperature, is the exciting 

 cause of electrical currents, which perpetually traversing the 

 earth's surface in a direction nearly parallel to the equator, give 

 to the earth "a constant magnetic polarity.' 5 Perhaps the more 



concernm 



magnet i 



electric currents circulating at or near the earth's surface, in- 

 duced by the heat of the sun. Prof. Barlow, who adopts this 

 view, conceives that only one link is wanting to complete the ex- 

 planation of terrestrial magnetism, viz. the discovery of the me- 



apparatus. Brewster remarks 



action 



? 



ble of developing magnetism in particles such as those which are 

 known to constitute our globe, the great difficulty would be re- 

 moved." 



In seeking for the explanation of the connection between the 

 magnetic and thermal phenomena of the earth, philosophers seem 

 hitherto to have regarded the heat as only modifying in some in- 



