TERRESTRIAL MAGNETISM. 179 



quantitative m the laws of one of the great phsenomena of 

 nature. We may therefore justly hope, that these laws, when 

 compared with those which prevail in the atmosphere and 

 in still more distant spaces, will gradually conduct us nearer 

 to the genetical explanation of the magnetic forces. As 

 yet we can only boast of having opened a greater number of 

 paths which may possibly lead to such explanation. In the 

 physical theory of terrestrial magnetism (which must not be 

 confounded with its purely mathematical theory), as in that 

 of the meteorological processes of the atmosphere, a prema- 

 ture satisfaction can only be obtained by those who permit 

 themselves to set aside as erroneous, all those phenomena 

 which are inconsistent with their own views. ( 166 ) 



Telluric magnetism, and the electro-dynamic forces mea- 

 sured by the ingenious Ampere, ( l67 ) are intimately con- 

 nected both with the terrestrial or polar light (Aurora), and 

 with the external and internal temperature of our planet, 

 whose magnetic poleshave been regarded by some philosophers 

 as poles of cold. ( 168 ) That which, more than 130 years ago, 

 Halley ( 16 9) put forward as a bold conjecture, viz. that the 

 Aurora is a magnetic phsenomerion, has, by Faraday's 

 brilliant discovery of the evolution of light by the action of 

 magnetic forces, been raised from a mere conjecture to an 

 experimental certainty. There are precursors of the Aurora ; 

 the luminous nocturnal appearance is usually foretold by 

 antecedent irregularity in the diurnal inarch of the magnetic 

 needle, indicating a disturbance in the equilibrium of the 

 distribution of the Earth's magnetism. When the disturb- 

 ance has reached a great degree of intensity, the equilibrium 

 is restored by a discharge accompanied by an evolution of 



