d yet within a page of the
place the effect is considered as of the same kind with that produced in
iron.

[A] Edin. Phil. Journal, 1825, p. 124.

250. I had two such plates mounted, one of copper, one of iron. The copper
plate alone gave sixty vibrations, in the average of several experiments,
before the arc of vibration was reduced from one constant mark to another.
On placing opposite magnetic poles near to, and on each side of, the same
place, the vibrations were reduced to fifteen. On putting similar poles on
each side of it, they rose to fifty; and on placing two pieces of wood of
equal size with the poles equally near, they became fifty-two. So that,
when similar poles were used, the magnetic effect was little or none, (the
obstruction being due to the confinement of the air, rather,) whilst with
opposite poles it was the greatest possible. When a pole was presented to
the edge of the plate, no retardation occurred.

251. The iron plate alone made thirty-two vibrations, whilst the arc of
vibration diminished a certain quantity. On presenting a magnetic pole to
the edge of the plate (247.), the vibrations were diminished to eleven; and
when the pole was about half an inch from the edge, to five.

252. When the marked pole was put at the side of the iron plate at a
certain distance, the number of vibrations was only five. When the marked
pole of the second bar was put on the opposite side of the plate at the
same distance (250.), the vibrations were reduced to two. But when the
second pole was an unmarked one, yet occupying exactly the same position,
the vibrations rose to twenty-two. By removing the stronger of these two
opposite poles a little way from the plate, the vibrations increased to
thirty-one, or nearly the original number. But on removing it _altogether_,
they fell to between five and six.

253. Nothing can be more clear, therefore, than that with iron, and bodies
admitting of ordinary magnetic induction, _opposite_ poles on opposite
sides of the edge of the plate neutralize each other's effect, whilst
_similar_ poles exalt the action; a single pole end on is also sufficient.
But with copper, and substances not sensible to ordinary magnetic
impressions, _similar_ poles on opposite sides of the plate neutralize each
other; _opposite_ poles exalt the action; and a single pole at the edge or
end on does nothing.

254. Nothing can more completely show the thorough independence of the
effects obtaine