108 Mr, Christie on the Magnetism of Iron [Aug. 



repeated for the points marked 60, 120, 180, 240, 300. The 

 plate was now made to revolve in the contrary direction, and 

 similar observations made of the dip of the needle when the 

 several points 300, 240, 180, 120, 60, 0, coincided with the 

 upper part of the line parallel to the magnetic axis. Continuing 

 the revolution of the plate in this direction, a second set of 

 observations of the dip were made for the several points from 

 300 to 0. After this, the plate was again made to revolve in its 

 first direction, and a second set of observations made of the dip 

 for the points from to 300. I considered the mean of all the 

 observations in the two sets, when the plate revolved from to 

 300, as the mean dip when the plate revolved in this direction ; 

 the mean of all the observations in the two sets, when the plate 

 revolved from 300 to 0, as the mean dip when the plate revolved 

 in this direction ; and the difference between these mean dips as 

 the deviation due to the rotation of the plate. -* 



"As 1 had experienced that the dipping needle, even when of 

 the best construction, was an instrument from which accurate 

 results could only be obtained by taking a mean of a great 

 number of observations, I was aware that, by making only two 

 for each point of the plate, I was liable to an error in the obser- 

 vations for each point taken separately, but this I considered 

 would be counteracted in taking the mean for all the points; 

 so that the mean results could not err far from the truth. The < 

 dipping needle which I made use of was a very good instrument,^ 

 by Jones, of Charing Cross : the needle, made according to 

 Captain Kater's construction, consisted of two arcs of a circle; 

 its length was 7 inches. The plate was the same I had used 

 in the experiments with the horizontal needle. 



" For the better distinguishing of the edges of the plate and 

 the direction of its rotation, I conceive two planes at right 

 angles to each other to pass through its centre ; one, the plane 

 of the equator or a plane parallel to it, which I call the equa- 

 torial plane ; the other, the plane of the secondary to the 

 equator and meridian, or a plane parallel to this secondary, 

 which I call the plane of or parallel to the axis. The inter- 

 sections of the first plane \^ith the edges of the plate, 1 call 

 the equatorial north and south edges ; and the intersections of 

 the second, the polar north and south edges. 



** From the observations thus made it appears that, in this posi- 

 tion of the plate, the deviation of the upper, or south end of the 

 needle, due to rotation, was in the direction in which the north or 

 lower edge of the plate revolved, and the deviation of the north 

 or lower end of the needle, in the direction of the rotation of the 

 upper or south edge of the plate. It would follow from this, 

 that if a needle could be suspended freely by its centre of 

 gravity, and the centre of the plate were in longitude 90°, 

 latitude 0°, and its plane at right angles to the meridian; 



