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in the same direction as the disc. Sir John Herschel and 
Mr Babbage* soon afterwards tried the converse experiment 
of causing the magnet to rotate parallel to a suspended disc, 
when the disc commenced to rotate in the same direction 
as the magnet. Now in both these cases if the suspended 
body were prevented from rotating, a reaction tending to de- 
stroy its rotation would take place upon the rotating body. 
The science of magneto-electricity being unknown at the time 
of these discoveries, they were inexplicable from previously 
known properties of magnetic and non-magnetic bodies. We 
now know” that when a diamagnetic body is set in motion 
near a magnet, that electrical currents arise in the moving 
body and by Cirsted’s discovery, the magnet tends to set itself 
at right angles to the current thus formed, and so goes in 
Arago’s experiment into rotation; and if prevented doing so, 
it reacts on the rotating body to destroy its rotation. 
The converse experiment to the above is a very impressive 
one and more directly applying to our present subject. It is 
the immediate and sudden destruction of rotatory motion in 
an angular diamagnetic body rotating near the poles of a 
powertul electro-magnet, when the current is formed. The 
sudden destruction of the momentum in a heavy rotating 
square prism of copper in such a case surprises us when first 
seen, from there being no evident force to produce it. 
Now in the case of the magnetic earth and diamagnetic 
moon, the same forces but in different degrees must arise, and 
if the moon ever possessed or received a rotatory motion, 
relatwe to the earth, it would gradually but surely cease. 
The moon’s absolute rotatory motion in her synodic period 
forms a no rotatory motion with respect to the earth; which 
is the point involved in the magneto-electric phenomena. 
Some years ago there was an active controversy as to whether 
1 See their paper read before the Royal Society, 16th June, 1825. 
* See Faraday’s paper read before the Royal Society, 24th Nov. 1831. 
