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manner as a dipping needle is poised; and suppose two other magnets to be placed perpendicularly, one under each pole of the horizontal magnet, and a little below it, with their north poles uppermost; then it is evident that the south pole of the horizontal magnet will be attracted by the north pole of one of the perpendicular magnets, and its north pole repelled by the north pole of the other; in this state it will remain at rest, but if by any means we reverse the polarity of the horizontal magnet, its position will be changed and the extremity, which was before attracted, will now be repelled, if the polarity be again reversed, the position will again be changed, and so on indefinitely. To produce, therefore, a continued vibration, it is only necessary to introduce into this arrangement some means by which the polarity of the horizontal magnet can be instantly changed, and that, too, by a cause which shall be put in operation by the motion of the magnet itself; how this can be effected