of intensity in the dipping and horizontal needles , &c. 183 
Therefore, as has been stated, the change of intensity in 
the horizontal needle is due, principally, to a daily variation 
in the amount of the dip ; not to a real change of intensity 
in the terrestrial magnetic force. This at least appears 
to be a legitimate deduction from the preceding observations ; 
from which circumstance, and that of the daily variation in 
the direction of the horizontal needle, we are naturally led to 
the conception of a small variation in position of the magnetic 
axis, corresponding to a revolution of the polar point round 
its mean position as a centre, produced by the action of the 
sun, on the magnetism of the parts of the earth, successively 
exposed to its influence. And, moreover, it seems by no 
means improbable, that the annual variation of the position 
of the magnetic pole may ultimately be traced to the same 
universal cause. 
I have not attempted to enter into any minute calculations 
on this subject, but I believe it will be found, that if the radius 
of the circle, described by the pole of the general magnetic 
axis of the earth during the day, be supposed to subtend at 
the centre an angle of 2 or sj- minutes, it will reconcile, to a 
considerable degree of precision, nearly all the observations 
on the daily variation of the direction, and daily change of 
intensity of the horizontal needle, made both in Europe and 
within the Arctic Circle. If, also, we suppose the magnetic 
north pole, during the passage of the sun over its meridian, 
when lying between the pole of the world and the sun, to 
advance more to the westward, or in a direction contrary to 
the rotation of the earth on its axis, than it returns to the 
eastward, or in the direction of rotation of the earth during 
the sun's passage over the opposite meridian, when the pole 
