490 • Mr. Babbage and Mr. Herschel's account of the 
magnet revolving beneath it. The latter is that of a com- 
pass needle, or of our neutralised system of vertical magnets 
suspended over a revolving disc of copper. A very pretty 
illustration of the direction of these forces is obtained by 
suspending a circular disc of zinc or copper from the end of 
a counterbalanced arm, which is itself suspended by its 
middle, thus constituting a kind of double balance of torsion. 
If the length of the arm be so adjusted, that the circumfer- 
ence of the disc shall be an exterior tangent to the circle de- 
scribed by the poles of a revolving magnet, the whole disc 
will be swept round in an orbit concentric with the motion of 
the magnet, while it at the same time acquires a rotatory 
motion on its own centre in the contrary sense. The centri- 
fugal force is here overcome by the arm and the weight of 
the disc, and the velocity goes on accelerating till the increase 
of resistance puts a stop to further accessions. 
33 . In Mr. Barlow's experiments, the earth is our induc- 
ing magnet ; its two poles both act on every particle of the 
revolving shell employed in that gentleman's experiments, 
and their action when complete produces two poles, a north 
and a south, at opposite extremities of the diameter parallel 
to the dip. This is the case when the shell is at rest. Let 
it now be set in motion about any axis, anyhow inclined .to 
the dip. If the communication and loss of magnetism were 
instantaneous, the places of the poles (i. e. the points of 
maximum polarity ) would be unaffected by the rotation ; but 
as that is not the case, these points, in virtue of the principles 
already stated, will shift their places, and decline from the 
direction of the dip in the same direction as the shell's mor- 
tion, that is to say, in the direction of a tangent to a small 
