_Of Magnetism._
Magnetism is a property peculiar to iron, or some ores of it. The earth
itself, owing probably to the iron ores contained in it, has the same
property. But though all iron is acted upon by magnetism, _steel_ only
is capable of having the power communicated to it.
Every magnet has two poles, denominated _north_ and _south_, each of
which attracts the other, and repels that of the same kind with itself.
If a magnet be cut into two parts, between the two poles, it will make
two magnets, the parts that were contiguous becoming opposite poles.
Though the poles of a magnet are denominated _north_ and _south_, they
do not constantly, and in all parts of the earth, point due north or
south, but in most places to the east or west of them, and with a
considerable variation in a course of time. Also a magnet exactly
balanced at its center will have a declination from an horizontal
position of about 70 degrees. The former is called the _variation_, and
the latter the _dipping_ of the magnetic needle.
A straight bar of iron which has been long fixed in a vertical position,
will become a magnet, the lower end becoming a north pole, and the upper
end a south one; for if it be suspended horizontally, the lower end will
point towards the north, and the upper end towards the south. Also any
bar of iron, not magnetical, held in a vertical position, will become a
temporary magnet, the lower end becoming a north pole, and the upper end
a south one; and a few strokes of a hammer will fix the poles for a
short time, though the position of the ends be changed. Magnetism may
likewise be given to a bar of iron by placing it firmly in the position
of the dipping-needle, and rubbing it hard one way with a polished steel
instrument. Iron will also become magnetical by ignition and quenching
it in water in the position of the dipping-needle.
Magnetism acts, without any diminution of its force, through any medium;
and iron not magnetical will have that power while it is in connexion
with a magnet, or rather the power of the magnet is extended through the
iron.
Steel filings gently thrown upon a magnet, adhere to it in a curious
manner; and the filings, acquiring magnetism by the contact, adhere
together, and form a number of small magnets, which arrange themselves
according to the attraction of the poles of the original magnet. This
experiment is made to the most advantage upon a piece of pasteboard, or
paper, placed over the magnet.
Magnetism is communicated by the friction, or the near position, of a
magnet to a piece of steel of a size less than it. For this reason a
combination of magnetical bars will have a greater effect than a single
one; and in the following manner, beginning without any magnetism at
all, the greatest quantity may be procured. Six bars of steel may be
rendered slightly magnetical by fixing each of them successively to an
upright poker, and stroking it several times from the bottom to the top
with the lower end of an old pair of tongs. If then four of these bars
be joined, the magnetism of the remaining two will be much increased by
a proper method of rubbing with them; and by changing their places,
joining the strongest, and acting upon the weakest, they may all be made
as magnetical as they are capable of being.
The strength of a natural magnet may be increased by covering its polar
extremities with steel. This is called the _arming_ of the loadstone.
To account for the variation of the needle, Dr. Halley supposed the
earth to consist of two parts, an external _shell_ and an internal
_nucleus_, detached, and having a revolution distinct from it; and that
the action of the poles of the shell and of the nucleus would explain
all the varieties in the position of the needle. But others think that
the cause of the magnetism of the earth is not _within_, but _without_
itself. One reason for this opinion is, that a magnet is liable to be
affected by a strong aurora borealis; and another is, that the variation
of the needle proceeds in such manner as supposes that the motion of the
nucleus must be quicker than that of the shell of the earth; whereas,
since it is most natural to suppose that motion was communicated to the
nucleus by the shell, it would be slower.
Some idea of the quantity and the progress of the variation of the
needle may be formed from the following facts.--At the Cape of Good
Hope, when it was discovered by the Portuguese, in 1486, there was no
variation, the needle there pointing due north; in 1622 it was about 2
degrees westward, in 1675 it was 8 deg. W. in 1700 about 11 deg. W. in 1756
about 18 deg. W. and in 1774 about 21-1/2 deg. W. In London, in 1580, the variation
was 11 degrees 15 seconds E.; in 1622 it was 6 deg. E. in 1634 it was 4 deg.
5 min. E. in 1657 it was nothing at all; in 1672 it was 2 deg. 30 min.
W. in 1692 it was 6 deg. W. in 1753 it was about 16 W. and at present it
is about 21 W.
The longitude may in some places be found by the variation of the
needle; and Mr. Churchman, of America, having given much attention to
the subject, comparing the observations of others, and many of his own,
thinks that he has found a method of determining the longitude to a
great degree of certainty, in most cases, by this means.
He says there are two magnetic poles of the earth, one to the north and
the other to the south, at different distances from the poles of the
earth, and revolving in different times; and from the combined influence
of these two poles he deduces rules for the position of the needle in
all places of the earth, and at all times, past, present, or to come.
The north magnetic pole, he says, makes a complete revolution in 426
years, 77 days, 9 hours, and the south pole in about 5459 years. In the
beginning of the year 1777 the north magnetic pole was in 76 deg. 4 min.
north latitude; and in longitude from Greenwich 140 deg. east; and the
south was in 72 deg. south latitude, and 140 deg. east from Greenwich.