364 Mr. Christie on the magnetism of 
The differences which we have noticed in the deviations ob- 
served at the same distance from the equator, is not more 
than I have found to arise from a slight change in the adjust- 
ment of the centre of the needle to the centre of the instru- 
ment, the plate remaining in the same position. These 
errors of adjustment I found it almost impossible to avoid, 
owing probably in a great measure to the magnetic centre of 
the needle not being in the centre of suspension ; and it was 
to counteract their effects, that I generally made observa- 
tions on contrary sides of the centre. 
With respect to the direction in which the deviation due to 
rotation took place, it appears, that the rotation of the plate 
always caused the north end of the needle to move in the 
same direction as the edge of the plate nearest the south pole 
of the magnetic sphere : so that the deviation of the 7 iorth end 
of the needle was in the direction in which the south edge of 
the plate moved, and that of the south end of the needle in the 
direction in which the north edge moved, referring the edges 
to the poles of the sphere. 
Having ascertained, that when the centre of the plate was 
in the pole, and its plane parallel to the equator, the deviation 
due to rotation was nothing ; and some of the first experiments 
which I had made having indicated that this was also the 
case when the centre of the plate was in the secondary to the 
equator and meridian, and its plane, as before, a tangent to 
the sphere, I wished to ascertain whether such were really 
the fact. The experiments,' the results of which are given 
in the following table, left no doubt in my mind on the sub- 
ject. In making them, the instrument was adjusted, so that 
the index at K (Fig. i ) pointed to zero, that at G to 19^30' 
