1826.] arising from its Rotation. 107 



the centre of the horizontal needle, my next experiments were 

 undertaken with the view of ascertaining whether the effects 

 produced by the rotation of the plate on the dipping 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 s n (fig. 1.) It was therefore 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 deviations of the dipping needle 

 would be less than those of the horizontal needle nearly in the 

 ratio of sin. 19° 30' 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 perpendicular 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, therefore, to ascertain the 

 deviations of a needle suspended freely by its centre of gravity, 

 corresponding to those of an horizontal 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 

 as in fig. 1, the compass being however removed ; the indexes 

 at 0, 0' were brought to M, &y on the moveable limb, and that 

 limb was placed at right angles to the fixed limb, so that the 

 plane of the plate was parallel to the magnetic meridian. The 

 dipping needle was then placed as nearly as possible in the 

 required position, and the levels being carefully adjusted, the 

 needle was made to vibrate freely andleft to settle. After the 

 plate had been made to revolve several times in the same di- 

 rection, the point marked was brought to coincide with the 

 upper part of a line parallel to the magnetic axis, and passing 

 through the centre of the plate. The needle was then shghtly 

 agitated, or made to vibrate through a small arc; and when it 

 settled, the dip was noted both at the upper and lower ex- 

 tremity, Qv the south and north end of the needle. This was 



