||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 zodiacal 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
distinction. 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.