356 Mr. Christie on the magnetism of 
here give the details, as I afterwards repeated them in a more 
convenient manner, and with greater precision; but shall 
merely point out the nature of them in general, and the con- 
clusions which I at the time drew from them. The instru- 
ment being adjusted, and the arm fixed so that the centre of 
the plate was in the position which I required, I made the 
plate revolve so that its upper edge moved from west to east, 
and noted the greatest and least deviation of the north end 
of the needle ; I then made the corresponding observations 
when the plate revolved in the contrary direction : a mean 
of the differences^ between the two greatest and between the 
two least I considered as the effect produced on the needle 
by the rotation of the plate in opposite directions. Repeating 
these in a variety of positions, I found that when the centre 
of the plate was in the magnetic meridian, its plane being 
always a tangent to the sphere circumscribed about the 
centre of the needle, the deviation of the needle caused by 
the rotation of the plate in its plane was the greatest when 
the centre of the plate was in the equator, and that it de- 
creased from there towards the poles, where it was nothing ; * 
that when its centre was on the equator, this deviation was 
the greatest when the centre of the plate was on the me- 
ridian, or in longitude 90°, and decreased to nothing in the 
east and west points, or when the longitude of the plate was 
0° or 180°; and that when the centre of the plate was in the 
* I should here mention, that, from the nature of my instrument, I could not 
make observations at the north pole ; but as the results, as far as I could observe, 
were of the same nature on this side of the equator as on the south side, I think I 
am warranted in concluding, that at the north pole the results would likewise be of 
the same nature as at the south pole. 
