, 'Meteor.', p.
584).
[footnote] **Saint-Martin, in the learned notes to Lebeau, 'Hist. du Bas
Empire', t. ix., p. 401.
[footnote] ***Humboldt, 'Asie Centrale', t. ii., p. 110-118. In regard to
the difference between agitation of the surface and of the strata lying
beneath it, see Gay-Lussac, in the 'Annales de Chimie et de Physique', t.
xxii., p. 429.
When the circles of commotion intersect one another -- when, for instance,
an elevated plain lies between two volcanoes simultaneously in a state of
eruption, several wave-systems may exist together, as in fluids, and not
mutually disturb one another. We may even suppose 'interference'
p 205
to exist here, as in the intersecting waves of sound. The extent of the
propagated waves of commotion will be increased on the upper surface of the
earth, according to the general law of mechanics, by which, on the
transmission of motion in elastic bodies, the stratum lying free on the one
side endeavors to separate itself from the other strata.
Waves of commotion have been investigated by means of the pendulum and the
seismometer* with tolerable accuracy in respect to their direction and total
intensity, but by no means with reference to the internal nature of their
alternations and their periodic intumescence.
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