SI 6 Mr. Christie on the magnetism of 
it is always desirable, that the observations should be made 
when the iron is at such a distance from the centre of the 
needle, that the effects of particular points, near its edges, on 
the ends of the needle are nearly insensible. Taking these 
circumstances into consideration, I was quite satisfied from 
these experiments, that, if the centre of the plate be in the 
secondary to the equator and meridian, and its plane per- 
pendicular to the plane of that circle, the rotation of the 
plate will produce no effect on the absolute deviations caused 
by the mass. 
In order to determine what effects would be produced by 
the rotation of the plate when its centre was in the secondary 
to the equator and meridian, and its plane in the plane of this 
circle, the instrument was adjusted as in fig. i. the index at 
g pointing to 70° 30'; the limb SJEl^ was then placed at 
right angles to SQN, and the arm AB attached to it with 
the iron plate on the axis as in fig. 5 ; and that the centre of 
the needle might be in the plane of the plate, the compass 
box was moved in the direction of the meridian. 
Some of my first observations were made with the centre 
of the plate in the equator, and I immediately found, that the 
deviation due to rotation^ instead of being o, as in the cases 
when the plate revolved in the planes at right angles to its 
present position, was here considerable ; and also that, that of 
the south end of the needle was in the direction of the upper, or 
south edge of the plate, contrary to what had been observed 
in the same plane at the pole (Table IV. lat. 90^). This in- 
dicated that there must be, at least, one point in this circle 
on each side of the pole, where the deviation due to rotation 
was o ; and to determine nearly the latitude of this point, I 
