70 Connexion between the Theory of the Earth and the 



the observed consequences relative to the secular motions of the 

 needle must of necessity follow. For the resultant (A) relative- 

 ly to the observer at any place being a fixed line, the resultant 

 (B) is movable, partaking as it must do of the westerly motion 

 of the internal mass. If then we describe a vertical circle through 

 the place of observation in the direction of the resultant (A) 

 the plane of this magnetic vertical is fixed, and twice during 

 each revolution of the fluid mass must the resultant (B) be found 

 in this plane. Commencing with its higher position in this 

 plane, it will pass westerly to its greatest elongation, thence east- 

 erly to its lower conjunction with the magnetic vertical, thence 

 to its greatest easterly elongation, thence westerly to its original 

 position. It is obvious that the needle freely suspended by its 

 centre of gravity, taking the direction of the diagonal between 

 the two resultants, will follow these motions of the resultant (B). 

 Leaving the magnetic vertical at its minimum dip, passing to its 

 greatest westerly declination, thence in its easterly progress pass- 

 ing said vertical at its maximum dip to its greatest declination 

 on the other side, thence to the place of beginning. The pole 

 of the dipping needle would thus describe a curve returning into 

 itself, with a period equal to that of the supposed westerly rev- 

 olution of the internal mass, and the secular motions of the ho- 

 rizontal needle in its arc of declination would have the same 

 period. 



It is obvious that if the magnetic poles of the external crust 

 and of the internal fluid mass, were coincident with the pole of 

 revolution, there would be no departure of the dipping or the ho- 

 rizontal needle from the plane of the meridian. As this coinci- 

 dence does not exist in fact, the position of the magnetic pole of 

 the solid crust will be determined by the intersection of magnetic 

 verticals for different places, the position of such vertical at any 

 place being determined by passing its plane through the needle 

 when its dip is at a maximum or minimum. One only of these 

 verticals will pass through the poles of revolution and coincide 

 with a terrestrial meridian ; and of course in all places situated 

 on this meridian, the dip of the needle will be at a maximum or 

 a minimum when the direction of the needle is due north and 

 south. In all other places on the earth's surface, there will be 

 an easterly or westerly declination of the needle when the dip is 

 at a maximum or minimum. 



