468 Mr. Babbage and Mr. Herschel's account of the 
corresponding with that of the rotation of the copper plate * 
and on employing instead of this bar, a very delicate azimuth 
compass, belonging to and the invention of Captain Kater, 
the influence of zinc, brass, and lead was similarly rendered 
sensible. 
In this first trial, having neither the command of a very 
rapid rotation, nor of massive metallic discs, the deviation of 
the compass observed did not exceed lo or ii degrees. In 
order therefore to enlarge the visible effect, and at the same 
time disencumber ourselves of the limit set to it by the 
polarity of the needle, it occurred to us to reverse the expe- 
riment, and ascertain whether discs of copper or other non- 
magnetic substances (in the usual acceptation of the word) 
might not be set in rotation if freely suspended over a re- 
volving magnet. In order to make this experiment, we 
mounted a powerful compound horse-shoe magnet, capable 
of lifting 20 pounds, in such a manner as to receive a rapid 
rotation about its axis of symmetry placed vertically, the 
line joining the poles being horizontal and the poles upwards.' 
A circular disc of copper, 6 inches in diameter and 0.05 inch 
thick, was suspended centrally over it by a silk thread with-' 
out torsion, just capable of supporting it. A sheet of paper 
properly stretched was interposed, and no sooner was the 
magnet set in rotation than the copper commenced revolving 
in the same direction, at fifst slowly, but with a velocity gra- 
dually and steadily accelerating. The motion of the magnet 
being reversed, the velocity of the copper was gradually 
destroyed ; it rested for an instant, and then immediately 
commenced revolving in the opposite direction, and so on 
alternately, as often as we pleased. 
