ears 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,