It is affected
instantly, but only transiently, by the distant northern light as it shoots
from the pole, flashing in beams of colored light across the heavens. 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 of time, in
p 178
every direction over the Earth's surface.*
[footnote] *There are also perturbations which are of a local character,
and do not extend themselves far, and are probably less deep-seated. Some
years ago I described a rare instance of this kind, in which an
extraordinary disturbance was felt in the mines at Freiberg, but was not
perceptible at Berlin. ('Lettre de M. de Humboldt a Son Altesse Royale le
Duc de Sussex sur les moyens propres a perfectionner la Connaissance du
Magnetisme Terrestre', in Becquerel's 'Traite Experimental de l'Electricite'
t. vii., p. 442.) Magnetic storms which were simultaneously felt from
Sicily to Upsala, did not extend from Upsala to Alten. (Gauss and Weber,
'Resultate des Magnet. Vereins', 1839, 128; Lloyd, in the 'Comptes Rendus de
l'Acad. des Sciences', t.
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