The mysterious course of the magnetic needle is equally affected by
time and space, by the sun’s course, and by changes of place on the
earth’s surface. Between the tropics the hour of the day may be known
by the direction of the needle as well as by the oscillations of the
barometer. It is affected instantly, but transiently, by the northern
light.
When the uniform horary motion of the needle is disturbed by a magnetic
storm, the perturbation manifests itself _simultaneously_, in the
strictest sense of the word, over hundreds and thousands of miles of
sea and land, or propagates itself by degrees in short intervals every
where over the earth’s surface.
Among numerous examples of perturbations occurring simultaneously and
extending over wide portions of the earth’s surface, one of the most
remarkable is that of September 25th, 1841, which was observed at
Toronto in Canada, at the Cape of Good Hope, at Prague, and partially
in Van Diemen’s Land. Sabine adds, “The English Sunday, on which it is
deemed sinful, after midnight on Saturday, to register an observation,
and to follow out the great phenomena of creation in their perfect
development, interrupted the observation in Van Diemen’s Land, where,
in consequence of the difference of the longitude, the magnetic storm
fell on Sunday.”
It is but justice to add, that to the direct instrumentality of the
British Association we are indebted for this system of observation,
which would not have been possible without some such machinery
for concerted action. It being known that the magnetic needle is
subject to oscillations, the nature, the periods, and the laws
of which were unascertained, under the direction of a committee
of the Association _magnetic observatories_ were established in
various places for investigating these strange disturbances. As
might have been anticipated, regularly recurring perturbations were
noted, depending on the hour of the day and the season of the year.
Magnetic storms were observed to sweep simultaneously over the
whole face of the earth, and these too have now been ascertained to
follow certain periodic laws.
But the most startling result of the combined magnetic observations
is the discovery of marked perturbations recurring at intervals of
ten years; a period which seemed to have no analogy to any thing
in the universe, but which M. Schwabe has found to correspond
with the variation of the spots on the sun, both attaining their
maximum and minimum developments at the same time. Here, for the
present, the discovery stops; but that which is now an unexplained
coincidence may hereafter supply the key to the nature and source
of Terrestrial Magnetism: or, as Dr. Lloyd observes, this system of
magnetic observation has gone beyond our globe, and opened a new
range for inquiry, by showing us that this wondrous agent has power
in other parts of the solar system.