THE AURORA BOREALIS
If fire-balls were thought miraculous and portentous
in days of yore, what interpretation must needs have
been put upon that vastly more picturesque phenomenon,
the aurora? “Through all the city,” says the
Book of Maccabees, “for the space of almost forty days,
there were seen horsemen running in the air, in cloth
of gold, armed with lances, like a band of soldiers: and
troops of horsemen in array encountering and running
one against another, with shaking of shields and multitude
of pikes, and drawing of swords, and casting of
darts, and glittering of golden ornaments and harness.”
Dire omens these; and hardly less ominous the aurora
seemed to all succeeding generations that observed it
down well into the eighteenth century—as witness
the popular excitement in England in 1716 over the
brilliant aurora of that year, which became famous
through Halley's description.
But after 1752, when Franklin dethroned the lightning,
all spectacular meteors came to be regarded as
natural phenomena, the aurora among the rest. Franklin
explained the aurora—which was seen commonly
enough in the eighteenth century, though only recorded
once in the seventeenth—as due to the accumulation of
electricity on the surface of polar snows, and its discharge
to the equator through the upper atmosphere.
Erasmus Darwin suggested that the luminosity might
be due to the ignition of hydrogen, which was supposed
by many philosophers to form the upper atmosphere.
Dalton, who first measured the height of the aurora,
estimating it at about one hundred miles, thought the
phenomenon due to magnetism acting on ferruginous
particles in the air, and his explanation was perhaps the
most popular one at the beginning of the last century.
Since then a multitude of observers have studied the
aurora, but the scientific grasp has found it as elusive in
fact as it seems to casual observation, and its exact
nature is as undetermined to-day as it was a hundred
years ago. There has been no dearth of theories concerning
it, however. Blot, who studied it in the Shetland
Islands in 1817, thought it due to electrified
ferruginous dust, the origin of which he ascribed to
Icelandic volcanoes. Much more recently the idea of
ferruginous particles has been revived, their presence
being ascribed not to volcanoes, but to the meteorites
constantly being dissipated in the upper atmosphere.
Ferruginous dust, presumably of such origin, has been
found on the polar snows, as well as on the snows of
mountain-tops, but whether it could produce the phenomena
of auroras is at least an open question.
Other theorists have explained the aurora as due to
the accumulation of electricity on clouds or on spicules
of ice in the upper air. Yet others think it due merely
to the passage of electricity through rarefied air itself.
Humboldt considered the matter settled in yet another
way when Faraday showed, in 1831, that magnetism
may produce luminous effects. But perhaps the prevailing
theory of to-day assumes that the aurora is due
to a current of electricity generated at the equator and
passing through upper regions of space, to enter the
earth at the magnetic poles—simply reversing the
course which Franklin assumed.
The similarity of the auroral light to that generated
in a vacuum bulb by the passage of electricity lends
support to the long-standing supposition that the aurora
is of electrical origin, but the subject still awaits
complete elucidation. For once even that mystery-solver the spectroscope has been baffled, for the line it
sifts from the aurora is not matched by that of any
recognized substance. A like line is found in the zo-
diacal light, it is true, but this is of little aid, for the
zodiacal light, though thought by some astronomers to
be due to meteor swarms about the sun, is held to be,
on the whole, as mysterious as the aurora itself.
Whatever the exact nature of the aurora, it has long
been known to be intimately associated with the phenomena
of terrestrial magnetism. Whenever a brilliant
aurora is visible, the world is sure to be visited
with what Humboldt called a magnetic storm—a
“storm” which manifests itself to human senses in no
way whatsoever except by deflecting the magnetic
needle and conjuring with the electric wire. Such
magnetic storms are curiously associated also with
spots on the sun—just how no one has explained,
though the fact itself is unquestioned. Sun-spots, too,
seem directly linked with auroras, each of these phenomena
passing through periods of greatest and least
frequency in corresponding cycles of about eleven
years' duration.
It was suspected a full century ago by Herschel that
the variations in the number of sun-spots had a direct
effect upon terrestrial weather, and he attempted to
demonstrate it by using the price of wheat as a criterion
of climatic conditions, meantime making careful observation
of the sun-spots. Nothing very definite came
of his efforts in this direction, the subject being far too
complex to be determined without long periods of observation.
Latterly, however, meteorologists, particularly
in the tropics, are disposed to think they find
evidence of some such connection between sun-spots
and the weather as Herschel suspected. Indeed, Mr.
Meldrum declares that there is a positive coincidence
between periods of numerous sun-spots and seasons
of excessive rain in India.
That some such connection does exist seems intrinsically
probable. But the modern meteorologist,
learning wisdom of the past, is extremely cautious
about ascribing casual effects to astronomical phenomena.
He finds it hard to forget that until recently all
manner of climatic conditions were associated with
phases of the moon; that not so very long ago showers
of falling-stars were considered “prognostic” of certain
kinds of weather; and that the “equinoctial storm”
had been accepted as a verity by every one, until
the unfeeling hand of statistics banished it from the
earth.
Yet, on the other hand, it is easily within the possibilities
that the science of the future may reveal associations
between the weather and sun-spots, auroras,
and terrestrial magnetism that as yet are hardly
dreamed of. Until such time, however, these phenomena
must feel themselves very grudgingly admitted
to the inner circle of meteorology. More and
more this science concerns itself, in our age of concentration
and specialization, with weather and climate.
Its votaries no longer concern themselves with stars or
planets or comets or shooting-stars—once thought the
very essence of guides to weather wisdom; and they are
even looking askance at the moon, and asking her to
show cause why she also should not be excluded from
their domain. Equally little do they care for the interior
of the earth, since they have learned that the
central emanations of heat which Mairan imagined as a
main source of aerial warmth can claim no such dis-
tinction. Even such problems as why the magnetic
pole does not coincide with the geographical, and why
the force of terrestrial magnetism decreases from the
magnetic poles to the magnetic equator, as Humboldt
first discovered that it does, excite them only to
lukewarm interest; for magnetism, they say, is not
known to have any connection whatever with climate
or weather.
