354 Mr. Christie on the magnetism of 
ticularly to the situation of certain points on the iron plate 
with respect to the limb, since, with one point coinciding 
with it, the deviation of the needle, when the centre of the 
plate' was on the meridian, would be easterly, and with 
another point coinciding, westerly ; whereas had the iron 
possessed no partial magnetism, which was the case I wished 
to investigate, there would have been no deviation when its 
centre was on the meridian. My first object was to find 
what points on the plate must coincide with the limb, in 
order that the plate, when its centre was' on the meridian, 
should cause no deviation in the needle ; and it was in my 
attempts to effect this, which at first sight appears sufficiently 
easy, that I discovered' the leading feature in all the phse- 
nomena which I am about to describe. 
General description of the phcenomena arising from the rotation of 
an iron plate. 
In order to find the points which I have mentioned, 1 
adjusted the instrument so that the plane of the fixed limb 
was exactly in the magnetic meridian, and then brought the 
other limb into the same plane : the centre of the plate was 
then on the magnetic meridian, and its plane perpendicular 
to that plane, as represented in Fig. i. I now made the 
-plate revolve in its own plane about the axis B 6, and noted 
very carefully its effect on the needle. In doing this I found 
that if I placed the plate on the arm, so that a certain point, 
c for instance, coincided with the plane of the limb, the 
deviation was different when the same point, by the revolu- 
tion of the plate, coincided with the limb again. As it ap- 
peared by this that the revolution of the plate ’had an effect 
