366 Mr. Christie on the magnetism of 
The next experiments which I made, were with the view 
of determining whether the rotation of the plate would pro- 
duce any deviation, when its plane coincided with the equator. 
For this purpose an axis was fixed perpendicularly on the 
arm of the instrument in such a manner, that, when the 
plate revolved on it, its plane was parallel to the limb. This 
is represented in fig. 5 : AB is the arm, on the cylindrical 
part of which, B 6, is fixed perpendicularly to it the axis V v, 
on which the plate of iron, C c, here seen edgewise, revolves. 
A, Uy are the two flat pieces, having an opening between 
them for the limb of the instrument ; Z is the clamping 
screw, and Y y the circular rim to support the iron plate, 
which are not seen in fig, 1. 
In order to make these observations, it was necessary to 
adjust the whole instrument twice ; since the deviations for 
the longitudes 90° and 270° could not be observed with the 
same adjustment as those for the longitudes 0° and 180®. 
For the longitudes 90® and 270®, the axis of the instrument 
was horizontal and pointed east and west, as in fig. 3, and 
the moveable limb E AW revolved on the axis until its plane, 
and therefore also that of the iron plate, made an angle of 
90° 30' with the horizon, rising towards the north ; so that 
the compass being elevated until the centre of the needle was 
in the plane of the plate, the plate was then in the equator. 
For the other longitudes, the axis of the instrument was 
inclined to the horizon at an angle of 19° 30', and in the 
plane of the meridian, as in fig. 2, and the moveable limb 
adjusted at right angles to the fixed one: the compass was 
then elevated to coincide with the plane of the plate. 
In these experiments the distance of the centre of the iron 
