HYPOTHESIS OF A TERRESTRIAL MAGNET. 295 



The intensity of the force T at any point of the surface may be 

 expressed as a function of the force at the equator T e by the formula 



At the magnetic pole the intensity is 



it is therefore twice as great as at the equator. 



These two formulae are at any rate approximately in agreement 

 with observations made at a given moment over the whole surface 

 of the earth. 



The absolute magnetic moment of the earth rs may be obtained 

 simply by means of the equation 



TR 3 



in which R represents the earth's radius. 



The hypothesis of a terrestrial magnet was introduced into 

 science by Gilbert. The pole in the southern hemisphere received 

 the name of austral pole, and is evidently of the same kind as the 

 north pole of magnets ; the pole situated in the northern hemisphere 

 is of the same kind as the south pole of magnets. 



This conception of a terrestrial magnet has also led to the 

 designation of austral pole being applied to that pole of the needle 

 which turns towards the north, and of boreal pole to that which 

 turns towards the south; in the theory of fluids, in spite of the 

 contradiction, we may say that a north pole contains austral fluid 

 and a south pole boreal fluid. But it is better to abandon the 

 expressions austral and boreal, which may give rise to misconcep- 

 tions, and to call, as we have done, positive magnetism that which 

 corresponds to the north pole of magnets, and negative magnetism 

 that of the south pole. 



308. The hypothesis of an infinitely small central magnet is 

 only one of the forms under which the earth's magnetism may be 

 represented ; it is even that which is least probable, seeing that the 

 undoubtedly very high temperature of the centre of the earth is 

 incompatible with the existence of bodies strongly magnetised. 



We know, for instance, that two superficial hemispherical layers 

 equal and of opposite signs, distributed so as to produce a constant 



