384 Mr, Christie on the magnetism of 
needle itself corresponded with those which I had observed 
on the horizontal needle. In making these it was necessary 
to adjust the dipping needle on a stand detached from the 
instrument, on the arm of which the iron plate revolved, on 
account of the diameter of the case of the dipping needle 
being greater than the distance sn (fig. 1). It was there- 
fore only in particular positions that I could observe the 
deviation caused by the rotation of the plate. This however 
was of the less importance, since, as I expected that the de- 
viations of the dipping needle would be less than those of the 
horizontal needle nearly in the ratio of sin. 19° so* to 1, it 
was only in the cases in which they were the greatest that I 
was likely to have been able to observe them. 
As the dipping needle, when in the position of the dip, 
could only vibrate in the plane of the meridian, no effect 
corresponding to the deviations of the horizontal needle could 
be observed, either when the centre of the plate was in the 
intersection of the meridian and equator, and its plane per- 
pendicular to the planes of these circles, or when the centre 
of the plate was in the secondary to the meridian and equator, 
and its plane in the plane of this secondary. In order there- 
fore to ascertain the deviations of a needle suspended freely 
by its centre of gravity, corresponding to those of an hori- 
zontal needle, when the plate had those positions, and which 
I considered to be the principal points to be determined, it 
was necessary to observe the effect produced on the dipping 
needle when the centre of the plate was in the equator and 
exactly east or west of the centre of the needle, and its plane 
parallel to the plane of vibration of the needle; and also 
when its centre and plane were in the plane of vibration. 
In making these observations, the instrument was adjusted 
