ry and genial;[364] but a subsequent review of the subject in 1859
convinced him that no relation of any kind between the two kinds of
effects was traceable.[365] With the singular affection of our
atmosphere known as the Aurora Borealis (more properly Aurora Polaris)
the case was different. Here the Zurich Chronicles set Wolf on the right
track in leading him to associate such luminous manifestations with a
disturbed condition of the sun; since subsequent detailed observation
has exhibited the curve of auroral frequency as following with such
fidelity the jagged lines figuring to the eye the fluctuations of solar
and magnetic activity, as to leave no reasonable doubt that all three
rise and sink together under the influence of a common cause. As long
ago as 1716,[366] Halley had conjectured that the Northern Lights were
due to magnetic "effluvia," but there was no evidence on the subject
forthcoming until Hiorter observed at Upsala in 1741 their agitating
influence upon the magnetic needle. That the effect was no casual one
was made superabundantly clear by Arago's researches in 1819 and
subsequent years. Now both were perceived to be swayed by the same
obscure power of cosmical disturbance.

The sun is not the only one of the heavenly bodies by which the
magnetism of the earth is affected. Proofs of a similar kind of lunar
action were laid by Kreil in 1841 before the Bohemian Society of
Sciences, and with minor corrections were fully substantiated by
Sabine's more extended researches. It was thus ascertained that each
lunar day, or the interval of twenty-four hours and about fifty-four
minutes between two successive meridian passages of our satellite, is
marked by a perceptible, though very small, double oscillation of the
needle--two progressive movements from east to west, and two returns
from west to east.[367] Moreover, the lunar, like the solar influence
(as was proved in each case by Sabine's analysis of the Hobarton and
Toronto observations), extends to all three "magnetic elements,"
affecting not only the position of the horizontal or _declination_
needle, but also the dip and intensity. It seems not unreasonable to
attribute some portion of the same subtle power to the planets and even
to the stars, though with effects rendered imperceptible by distance.

We have now to speak of the discovery and application to the heavenly
bodies of a totally new method of investigation. Spectrum analysis may
be shortly des