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. Itis obvious that the needle freely suspended by its 
ceutre 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 are 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 magneti¢ 
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 oF 
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. 
a 
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