The advocate., 1897-08-18
What Causes the Aurora BOrealis2 2

Interpretation must needs have been
put upon that vastly more picturesque
phenomenon, the aurora? Through
all the city" says the Book of Macca-
bees, "fOr the space of almost forty
days, there were seen hOrsemen run
ming in the air, in cloth Of gold, armed
with lances, like a band of soldiers;
and troops of horsemen In array en-
countering and running one against
another, with shaking of shields and
multitude Of pixies, and drawing Of
swords, and casting Of darts, and gilt
terlng of golden ornaments and har-
ness" Dire omens these; and hardly
less OminOus the aurora seemed to all
succeeding generations that observed it
down till well into the eighteenth CEN
tury-as witness the popular excite
ment in England in 1716 over the bril-
liant aurora of that year, which became
famous through Halley's description.
But after 1852, when Franklin de-
thrOned 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 re-
corded once in the seventeenth-as due
to the accumulation Of electricity on
the surface of polar SHOWS, and it's dis
charge to the equator through the upper
atmosphere. Erasmus Darwin sug-
gested 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 hun
dred miles, thought the phenomenon
due to magnetism acting on ferrugin
Gus particles in the air, and his expla-
nation was perhaps the most popular
one at the beginning of the century.
Since then a multitude of observers
have studied the aurora, but the scien-
titic grasp has found it as elusive in
fact as it seems to casual observation,
and it's exact nature is as undetermined
to-day as it was hundred years ago.
There has been no dearth of theories
concerning it, however. Blot, who
studied it in the Shetland Islands in
1917, thought it due to electrified fer
ruginOus dust, the origin of which he
ascribed to Icelandic volcanoes. Much
more recently the idea of ferruginous
particles has been revived, their pres
ence 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
SHOWS, as we'll as on the SHOWS of
mountain tops, but whether it could
produce the phenomena of aurOras is
at least an open question. The prevail
ing theory of to-day is that the aurora
is due to a current of electricity gen.
Grated at the equator, and passing
through upper regions of space, to enter
the earth at the magnetic poles--sim-
ply reversing the course which Frank
lin assumed.
The similarity of the auroral light to
that generated in 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 eluci--
dation. For once even that mystery
solver, the spectroscope, has been bat
fled, for the line it Sits from the aurora
is not matched by that of any recog-
nized 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 phenOm-
ena of terrestrial magnetism. When
ever a brilliant aurora is visible, the
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