CHAPTER VIII.

The magic swan--Properties of the magnet--The mariners’
compass--The process of magnetizing--The dip of the
needle--Magnetic properties in various substances.


A magician of former days had a figure of a swan, which floated on a
vessel of water, round the rim of which were placed the twenty-four
letters of the alphabet. Addressing the spectators, he was accustomed
to ask for a name to be given him, and it was correctly spelt by the
swan, as it moved from one letter to another till it had indicated
the whole. A little philosophy, in this instance, produced repeatedly
great astonishment. A magnetic bar was placed in the swan, and the
performer had a powerful magnet concealed in his own dress, and the
swan, of course, followed his motions. Thus, if he wanted the swan to
spell “Selina,” he moved first to S, then to E, and so on, through
the successive letters of that name, till the word was spelt. On one
occasion, however, the performer was not a little disconcerted--the
swan stopped in its course and refused to move. Again and again the
effort was made, but it was utterly in vain; the magician could only
acknowledge that some person was in the room aware of his secret, and
counteracting his movements. Sir Francis Blake Delaval avowed himself
to be the person: he produced a magnet which he had used on facing the
performer as he stood at the table; the swan was, therefore, placed
between two attractive instruments, and, of course, remained immovable.

A magnet may be described as a piece of iron, which possesses the
property of turning towards the poles of the earth. This extraordinary
quality does not necessarily belong to all specimens of iron in its
native state, but only to one kind or variety called the oxide, on
account of its union with oxygen in a particular condition. The
possession of a special quality in this ore of iron was not discovered
from its polarity, or power of turning to the poles of the earth, but
from its property of attracting small pieces of iron, which are not
magnetic; and hence it was called the loadstone.

There are many uses to which the magnet has been applied, and there
is a probability of its being much more extensively employed; but its
most important application is in the construction of the mariners’
compass, which renders it possible freely to traverse the ocean. There
has been some controversy as to the discovery of the directive power
of the magnet, and the invention of the compass. It was once supposed
to have been unknown until about the thirteenth century, but it is
now generally acknowledged that the Chinese were acquainted with the
compass at least eleven hundred and fourteen years before the birth
of Christ. At the commencement of the thirteenth century, it was
certainly in use in Europe; for cardinal de Vitty mentions it with some
particularity, in a work entitled “The History of the East,” where he
says, “The iron needle, after contact with the loadstone, constantly
turns to the north star, which, as the axis of the firmament, remains
immovable, while the others revolve; and hence it is essentially
necessary to those navigating on the ocean.” This shows that the
compass was not invented in Europe, as commonly believed, by Gioia,
a pilot, and a native of Pasitano, a small village, situated near
Amalfi, who lived about the end of the thirteenth century, but, by
him, it appears to have been made fully available for the purposes of
navigation.

As used by sailors in the Mediterranean at that period, it was a very
uncertain guide; for the compass then consisted of a magnetic needle
attached to two straws on a piece of cork, floating on water in a
basin, or glass vase. Gioia, therefore, placed the magnetic needle
upon a pivot, so that it was free to move in any direction, and thus
prevented that inconvenience and inaccuracy of observation which must
have resulted from the motion of the needle floating on water, agitated
by the tossing of the vessel. The magnetic needle was afterwards
attached to a card divided into thirty-two points, called the _rose
des vents_, so that the direction in which a vessel was sailing
could be minutely determined, and the means of ascertaining it was no
longer dependent on the accuracy of the eye in measuring distances.
The mariners’ compass is still constructed in the same manner, but
is inclosed in a box with a glass cover, and is thus preserved from
the influence of the wind. Another improvement has been made in so
suspending the box that, however the vessel may be pitched by the
waves, and rolled from side to side, the needle remains in a horizontal
position, and gives accurate indications of the direction in which the
vessel is sailing.

In addition to the properties already mentioned, the loadstone has the
power of communicating its virtues to any piece of hard iron or steel,
and that, without diminution of strength; so that, if but one piece
had been discovered, it would have been sufficient for the production
of all the magnets that have ever been formed by man. Other means may
be adopted of accomplishing this purpose. Take a bar of iron, and,
striking it several times with a hammer, it will become magnetic. This
experiment may be performed with a common poker. The magnetism thus
communicated to a steel bar will be much increased in power, if it be
supported on another bar during the process of hammering.

Gay Lussac, a French chemist of great celebrity, discovered a method
of making magnets by a process so simple, that it may, in all cases,
be applied successfully. Take a piece of thin iron wire and suspend
it in a vertical position. The earth itself being a magnet, induces a
magnetic power in the wire. To render this permanent, twist the wire
till it breaks, and a magnet will be obtained.

Mrs. Somerville, well known for her excellent philosophical works,
made some experiments on the effect of solar light in the production
of permanent magnetism. If half of a small sewing needle be covered
with paper, and the exposed part be placed in the violet or indigo ray,
magnetism will be induced, and the same effect will be produced in a
smaller degree by the blue and green.

To describe but one more mode; magnets are readily made by what is
called the single touch, and this is perhaps the most simple and most
effective way of proceeding. Place the steel bar to be magnetized on
a table, or any other convenient place, and, as nearly as possible,
north and south, which position is called by philosophers, the magnetic
meridian. This being done, draw over it perpendicularly a strong
magnet. In this operation, it is necessary to begin at one end of the
bar, and draw the magnet over its entire length, and then again in the
same direction. It must not be drawn backward and forward, for the
power communicated in one direction, would be destroyed by an opposite
motion.

The following experiments are very instructive:--Suspend a magnetic
needle by a silk cord, so that it will hang in a horizontal position.
Then bring it over the centre of a large magnet lying upon a table,
and it will still retain its position; but, as it is brought near to
either end, it will be bent downwards, and, at the extremities, will
be vertical. This experiment illustrates what is called the dip of
the magnet. On the equator of the earth, the needle is horizontal,
or nearly so, but as it is brought near the poles it dips, and over
either magnetic pole would be vertical. The reason of this is evident
from the former experiment: at the equator, each pole of the needle is
attracted in an equal degree by the north and south poles of the earth;
but, if we proceed northward, the north pole of the magnet will be more
attracted than the south, and point towards it until at last it becomes
vertical. The poles of the earth’s rotation, that is, the points which
would form the terminations of its axis, did it revolve on one, are
not the magnetic poles; nor is the equator of the earth the magnetic
equator. They do not, however, greatly vary.

Take, also, a bar magnet, and, placing it upon a table, cover it with a
sheet of writing-paper. Then sprinkle upon it some fine iron filings,
and they will arrange themselves in very beautiful curves round the
magnet, showing, as it is supposed, the circulation of the magnetic
fluid. From this experiment, we learn that the magnetic power is
greatest at the poles; and this is true in reference to the magnetism
of the earth, which increases in power from the magnetic equator to
the magnetic poles of the earth, as determined by a great variety
of interesting and delicate experiments. Sir Graves C. Haughton has
communicated a paper to the June number of Brewster’s _Philosophical
Magazine_, entitled “Experiments proving the common nature of
Magnetism, Cohesion, Adhesion, and Viscosity.”

This paper contains two separate sets of experiments, the first of
which relates to the attraction the magnetic needle has for various
mineral, vegetable, and animal substances: and it is not a little
remarkable that antimony and bismuth, as well as copper, tin, and
cadmium, are, in these experiments, shown to have attractive powers
for the magnetic needle; though, in those made by Dr. Faraday, he has
ranged them amongst the class of dia-magnetics, that is, of those that
exhibited repulsion. Arsenic, too, which he found so intractable,
was made, in the present experiments, to assume the real magnetic
character, that is to say, the power of attracting and repelling, by
being kept for a short time in contact with a bar magnet. Iodine,
likewise, was found, on bringing it near the needle, to be able to
attract it.

In most of these experiments, the needle was made to attach itself to
the substances by being forced towards them by a magnet, which was
gently withdrawn after contact was so effected. In this way, and by a
reference to the degrees of the compass traversed by the needle, a hair
of the head, or a spark of diamond, can be accurately measured. The
strength of the needle in its movement on a pivot was ascertained by
azimuths, of which a detailed account is given.

The remainder of the memoir, which is contained in a supplementary
number of the Magazine, is devoted to a detail of about five hundred
experiments, in which non-ferruginous needles were made, by a
modification of the magnetic needle, of which they formed a portion,
to attach themselves to the same substances as in the preceding
experiments. Thus, for instance, needles of most of the remarkable
metals, as well as of glass, were found to have a strong affinity
for nearly every kind of substance, whether mineral, vegetable, or
animal, if its density was greater than that of cork or charcoal.
Brass surpassed all the metals in its power of attraction, and, what
is most remarkable, the magnetic needle was the lowest of all in the
scale, showing not much more than one-third of the attractive energy
of soft iron. Every substance of a crystalline or vitreous character
exhibited remarkable magnetic properties, and this could not be
mistaken, as it might be heightened at pleasure by contact with either
pole of a powerful magnet. Towards the close of the experiments, the
curious discovery was made, that needles of ivory, mother-of-pearl,
tortoise-shell, horn, etc., were singularly magnetic, and this is
traced to the albumen and gelatine they contained; and the inference
is drawn, from this and other facts, that the cohesive, adhesive, and
viscous properties of bodies are owing to real magnetic qualities,
and that, by drying, albuminous, gelatinous, and glutinous fluids
constitute various kinds of glass, which view is supported by what
takes place with the gelatinous hydrate of silicium.

“The preceding experiments,” says the writer, “include a vast variety
of substances in the mineral, vegetable, and animal kingdoms, that
exhibit such strong attractive affinities for one another, that,
however much they may differ in their external appearances, and in
their very natures, they are bound together by common bonds that
connect them all into a single family; for we find the metal attaching
itself to crystalline, animal, and vegetable substances; and, again,
the crystal, whether we call it by the name of diamond, salt, or
sugar-candy, connecting itself readily to metallic, animal, and
vegetable bodies. In a similar way, animal bodies attach themselves to
those that are mineral and vegetable; and, to complete the circle, the
vegetable kingdom, by its woods, its gums, its lac, and its resins, is
connected with them all.”