TERRESTRIAL MAGNETISM. 91 



discovery of the polar properties of the oxygen of the atmo- 

 sphere which surrounds our globe will present an element 

 in the explanation which shall then be given of the genesis 

 of that process. It is hardly conceivable that in the 

 harmonious concurrent action of all natural forces this 

 property of oxygen, and its modification by changes of 

 temperature, should have no share in calling forth magnetic 

 phenomena. If, in accordance with the conjecture expressed 

 by Newton, the substances belonging to the same planetary 

 system are, in all probability, for the most part the same,( 87 ) 

 we may also be led, by inductive reasoning, to surmise that 

 the endowment of gravitating matter with electro-magnetic 

 activity is not confined to our own globe. To assume the 

 contrary, would be to impose arbitrarily dogmatic limits on 

 cosmical views. Coulomb's hypothesis respecting the influ- 

 ence of the magnetic sun on the magnetic earth is certainly 

 not opposed to any analogy furnished by the knowledge we 

 have as yet acquired. 



If we now pass to the purely objective representation of 

 the magnetic phenomena presented to us by the Earth, at 

 the different parts of its surface, and in its different positions 

 relatively to the Sun, we must distinguish, in the numerical 

 results of observation, the variations which are included 

 within short periods, from those which extend over very long 

 periods. All are intermingled and intersecting, like circles 

 of undulation in fluids to which a movement is given, some- 

 times reinforcing, and sometimes partially or wholly com- 

 pensating and destroying each other. 



In the geographical distribution of the phsenomena, there 

 present themselves more particularly to our notice : 



