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I think it is desirable that this method of discussion 
should be extended to the phenomena round the magnetic 
equator. This magnetic equator may be regarded as 
approximately coincident with the terrestrial equator. It 
is the line all along which the freely suspended needle 
' points horizontally, just as the magnetic pole is the place at 
which the freely suspended needle points vertically down- 
wards. 
Now a little to the north of the magnetic equator we 
have, broadly speaking, the following phenomena : — 
(1) When the sun is north of the line, the influence of the 
sun upon the declination needle (as represented by that 
oscillation which culminates an hour or two after noon) 
tends to drive the north pole to the west. But when the 
sun is south of the line, this action becomes reversed, and 
drives the north pole eastwards. 
(2) Whether the sun is north or south of the line, its 
action upon the bifilar needle (as represented by that oscil- 
lation which culminates about noon) tends to increase the 
horizontal force. 
Now let us go a little to the south of the magnetic equa- 
tor, and we find the following behaviour 
(8) When the sun is south of the line, the influence upon 
the declination needle, represented as above, tends to drive 
the north pole to the east. But when the sun is north of 
the line, this action becomes reversed, and the north pole is 
driven westwards. 
(4) Whether the sun is north or south of the line, its 
action upon the bifilar needle, represented as above, shows 
that it tends to increase the horizontal force. 
It is, indeed, well known that there is a north hemisphere 
and a south hemisphere action of the sun upon the declina- 
tion needle, the one being the reverse of the other, and the 
southern limit of the first action being the northern limit of 
the second. And furthermore, this boundary line oscillates 
