383 
iron arising fro7n its rotation. 
io rotation are always such as would be produced by the sides 
of the equator of this imagmary dipping needle deviating in direc- 
tions contrary to the directions in which the edges of the plate 
move, that edge of the plate nearest to either edge of the 
equator producing the greatest effect on it. By referring to 
the particular laws which I deduced at the time of making 
the experiments in different planes, it will be seen that they 
are all comprised under this general law ; but this will be 
rendered more evident by taking an instance. 
When the centre of the plate is in the meridian, and its 
plane a tangent to the sphere, the eastern side of the equator 
of the imaginary dipping needle, according to the above law, 
will deviate in a direction contrary to that of the motion of 
the eastern edge of the plate, and consequently the northern 
extremity of the axis will deviate in a contrary direction to 
that of the motion of the plate's northern edge, or it will 
deviate in the direction in which the southern edge of the 
plate moves. Hence the horizontal needle obeying the 
deviations of this dipping needle, the deviations of its north 
end due to the rotation of the plate will be in the direction in 
which the south edge of the plate moves, which is the law 
deduced from the experiments, Table I, 
% 
Experiments with the dipping needle. 
Having found, in all the experiments which I have de- 
scribed, that the effects produced on the horizontal needle 
depended on the situation of the plate with respect to the 
axis and equator of an imaginary dipping needle passing 
through the centre of the horizontal needle, my next experi- 
ments were undertaken with the view of ascertaining whether 
the effects produced by the rotation of the plate on the dipping 
