u may perhaps learn all this in a single hour; but spend several at
it, if necessary; and remember, understanding it is not sufficient:
you must obtain a manual aptitude in addressing Nature. If you speak
to your fellow-man you are not entitled to use jargon. Bad
experiments are jargon addressed to Nature, and just as much to be
deprecated. Manual dexterity in illustrating the interaction of
magnetic poles is of the utmost importance at this stage of your
progress; and you must not neglect attaining this 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
endeavour to follow out: questions will arise which you will try to
answer. The same experiment may be twenty different 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. Should you ever become a patentee for the regulation of
ships' compasses, you will not fall, as some projectors have done,
into the error of screening off the magnetism of the ship by the
interposition of such substances.

If you wish to teach a class you must contrive that the effects 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 movements visible at a distance, denoting the north and south
poles by different colours, say green and red. You may also improve
upon your darning-needle. Take a strip of sheet steel, heat it to
vivid redness and plunge it into cold water. It is thereby hardened;
rendered, in fact, almost as brittle as glass. Six inches of this,
magnetised in the manner of the darning-needle, will be better able to
carry your paper indexes. Having secured such a strip, you proceed
thus:

Magnetise a small sewing-needle and determine its poles; or, break
half an inch, or an inch, off your magnetised 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