386 
Mr. Christie on the magnetism oj 
this direction ; and the difference between these mean dips as . 
the deviation due to the rotation of the plate. 
As I had experienced that the dipping needle, even when . 
of the best construction, was an instrument from which accu- 
rate 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 observations 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, con- 
sisted 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 })late 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 
equatorial 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 with the edges of the plate, I call 
the equatorial north and south edges ; and the intersections 
of the second, the polar north and south edges. 
In the following table, the numbers in the first column in- 
dicate the point on the plate which was in the polar south ; 
the inclinations of the needle corresponding to these positions 
