112 PHYSICAL GEOGRAPHY.

 

297. The Aurora Borealis, or northern light,
is a phenomenon of marvellous beauty, occurring
ia the sky of high latitudes in both the northern
and southern hemispheres. It appears in a va-
riety of forms; at times huge pillars of fire move
rapidly across the heavens, or the entire northern
sky is lighted as by a drifting storm of luminous
snow. The commonest appearance, however, is
that of an arch of fire, from which streamers
flash toward the zenith.

Auroras are most frequent in high latitudes,
though not in the immediate vicinity of the
poles.

Auroras are caused by the passage of electricity through
the rare air of the upper regions. The proofs are as fol-
lows: During the continuance of an aurora, the telegraph
wires show the presence of an unusual electrical disturb-
ance, and the magnetic needle is subject to frequent oscil-
lations; moreover, the same phenomena can be produced
by the passage of an electrical current through rarefied

gases, as in the Geissler tubes—different colors arising
from its passage through different gases,

 

 

 

 

 

 

 

 

 

 

   

 

Fig. 100, Aurora Borealis,

 

298. Magnetism.—The recent researches of
Herz leave little doubt that electro-magnetic phe-
nomena are due to a wave motion in the lumi-
niferous ether.

Magnets are bodies which have the power of
attracting particles of iron or the opposite poles
of other magnets.

All magnets possess an atmosphere of influence
surrounding them, called the magnetic field. The
magnetic field is traversed by lines of force, which
come out of the magnet at one point and enter it
at another, thus forming a magnetic cirewit. The

 

points where the lines come out are called poles;
the former being the positive or north pole, and
the latter the negative or south pole.

Magnets are either natural or artificial. Nat-
ural magnets are found in lodestone, a species of °
iron ore composed of oxygen and iron. Pieces
of hardened iron or steel may be magnetized, by
rubbing them with a lodestone, or by passing
electrical currents around them, thus forming
what are called electro-magnets. All magnetiz-
able substances become magnetized when they
are brought into a magnetic field.

If a magnetized bar or needle be suspended at
its centre of gravity so as to move freely in a
horizontal plane, after a few oscillations it will
come to rest, with one of its ends pointing nearly
to the geographical north pole of the earth. This
end of the magnet is called its north pole, the op-
posite end its south pole, and the magnet itself, a
magnetic needle.

 

Fig. 101, The Magnetic Needle,

299. Magnetic Attractions and Repulsions.—If a
magnet is brought near a magnetic needle, attraction or
repulsion will ensue—repulsion, when the poles are of the
same name; attraction, when they are of opposite names.
Thus, when a north pole is approached to a north pole, or
a south pole to a south pole, they repel each other; but
when a north pole is approached to a south pole, or a south
pole to a north pole, they attract. If the approaching
magnet is powerful, it will deflect the magnetic needle,
although several feet distant from it; and if placed per-
manently in this position, the magnetic needle will no
longer point to the north, but will turn toward the disturbing
magnet.

300. The Magnetic Properties of the Earth.—-
The Magnetic Needle—The magnetic needle
points to the north for the same reason that the
opposite poles of magnets point to each other
when they are sufficiently near. The entire earth
acts as one huge magnet, with its poles in the neigh-
borhood of the extremities of its avis, and the mag-
netic needle points toward these poles on account
of their attraction.