388 Mr. Christie on the magnetism of 
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 ; 
then also, the deviation of the south end of the needle due to 
rotation, would be in the direction of the north or lower edge 
of the plate, and the deviation of the north end, in the direc- 
tion in which the south or upper edge revolved ; which are 
precisely the directions of the deviations of the horizontal 
needle in this position of the plate. (See Table I.) 
The law which I have shown to obtain in all the experi- 
ments on the horizontal needle, viz. that the sides of the 
equator of the imaginary dipping needle always deviated in 
directions contrary to those in which the corresponding edges 
of the plate moved, I had derived previously to having an 
opportunity of making any experiments with the dipping 
needle : a comparison of the above results with this law will 
more fully illustrate its nature, and at the same time show 
their perfect accordance. In making this comparison, it is 
necessary to notice that, an increase of the dip of the needle, 
corresponds to a deviation of the southern edge of its equator 
towards the south pole, and of the northern edge towards the 
north pole ; and on the contrary, a diminution of the dip cor- 
responds to a deviation of the southern edge of the equator 
towards the north pole, and of the northern edge towards the 
south pole. Now, when the equatorial south edge of the 
plate revolved towards the polar south, and consequently the 
