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A LECTURE ON MAGNETISM. 377

we go to the equatorial regions of the earth with a suit-
ably-suspended needle, we shall find there the position of
the needle horizontal. If we sail north, one end of the
needle dips; if we sail south, the opposite end dips; and
over the north or south terrestrial magnetic pole the needle
sets vertical. The south magnetic pole has not yet been
found, but Sir James Ross discovered the north magnetic
pole on the 1st of June, 1831. In this manner we estab-
lish a complete parallelism between the action of the earth
and that of an ordinary magnet.

The terrestrial magnetic poles do not coincide with the
geographical ones ; nor does the earth’s magnetic equator
quite coincide with the geographical equator. The direc-
tion of the magnetic needle in London, which is called the
magnetic meridian, encloses an angle of 24: degrees with
the true astronomical meridian, this angle being called the
Declination of the needle for London. The north pole of
the needle now lies to the west of the true meridian; the
declination is westerly. In the year 1660, however, the
declination was nothing, while before that time it was
easterly. All this proves that the earth’s magnetic con-
stituents are gradually changing their distribution. This
change is very slow; it is technically called the secular
change, and the observation of it has not yet extended over'
a sufficient period of time to enable us to guess, even ap-
proximately, at its laws.

Having thus discovered, to some extent, the secret of
the earth’s power, we can turn it to account. I hold in my
hand a poker formed of good soft iron; it is now in the
line of dip, a tangent, in fact, to the earth’s line of magnetic
force. The earth, acting as a magnet, is at this moment
constraining the two fluids of the poker to separate, making
the loWer end of the poker a north pole, and the upper
end a south pole. Mark the experiment: I hold the knob
uppermost, and it attracts the north end of a magnetic

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