74 NOTES TO BOOK XII. 



It is one of the most curious results of this investiga- 

 tion that according to the most simple meaning which 

 we can give to the term "pole," 1 the earth has only two 

 magnetic poles ; that is, two points where the direction 

 of the magnetic force is vertical. And thus the isogonal 

 curves may be looked upon as deformations of the curves 

 deduced by Euler from the supposition of two poles, the 

 deformation arising from this, that the earth does not 

 contain a single definite magnet, but irregularly diffused 

 magnetical elements, which still have collectively a distant 

 resemblance to a single magnet. And instead of Han- 

 steen's Siberian pole, we have a Siberian region in which 

 the needles converge ; but if the apparent convergence be 

 pursued it nowhere comes to a point; and the like is the 

 case in the Antarctic region. When the 24 Gaussian 

 elements at any time are known the magnetic condition 

 of the globe is known, just as the mechanical condition 

 of the solar system is known, when we know the elements 

 of the orbits of the satellites and planets and the mass 

 of each. And the comparison of this magnetic condition 

 of the globe at distant periods of time cannot fail to sup- 

 ply materials for future researches and speculations with 

 regard to the agencies by which the condition of the 

 earth is determined. The condition of which we here 

 speak must necessarily be its mechanico-chemical condition, 

 being expressed, as it will be, in terms of the mechanico- 

 chemical sciences. The investigations I have been de- 

 scribing belong to the mechanical side of the subject ; but 

 when philosophers have to consider the causes of the 

 secular changes which are found to occur in this mecha- 

 nical condition, they cannot fail to be driven to electrical, 

 that is. chemical agencies and laws. 



