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364 FRAGMENTS OF SCIENCE.

power over your implements. As you proceed, moreover,
you will be tempted to do more than I can possibly suggest.
Thoughts will occur to you which you will endeavor to fol-
low out; questions will arise which you will try. to answer.
The same experiment may be twenty things to twenty
people. Having witnessed the action of pole on pole
through the air, you will perhaps try whether the magnetic
power is not to be screened off. You use plates of glass,
wood, slate, pasteboard, or gutta-percha, but find them all
pervious to this wondrous force. One magnetic pole acts
upon another through these bodies as if they were not
present. And should you become a patentee for the regu-
lation of ships’ compasses, you will not fall, as some pro-
jectors have done, into the error of screening ofi‘ the mag-
netism of the ship by the interposition of such substances.

If you wish to teach a class you must contrive that the
efi’ects which. you have thus far witnessed for yourself shall
be witnessed by twenty or thirty pupils. And here your
private ingenuity must come into play. You will attach
bits of paper to your needles, so as to render their move-
ments visible at a distance, denoting the north and south
poles by different colors, say green and red. You may also
improve upon your darning-needle. Take a strip of sheet-
steel, the rib of a lady’s stays will answer, heat it to vivid
redness and plunge it into cold water. It is thereby hard-
ened, rendered, in fact, almost as brittle as glass. Six
inches of this, magnetized in the manner of the darning-
needle, will be better able to carry your paper indexes.
Having secured such a strip, you proceed thus:

Magnetize a small sewing-needle and determine its
poles; or, break half an inch or an inch of? your magnetized
darning-needle, and suspend it by a fine silk fibre. The
sewing-needle or the fragment of the darning—needle is now
to be used as a test-needle to examine the distribution of
the magnetism in your strip of steel. Hold the strip up-

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