Physical geography. By Mary Somerville ...

158 PHYSICAL GEOG(IRAPHY. CHAP. XIII.
Three other series of undulations are formed simultaneously with
the preceding, by which the sound of the explosion is conveyed
through the earth, the ocean, and the air, with different velocities.
That through the earth travels at the rate of from 7000 to 10,000
feet in a second in hard rock, somewhat less in looser materials, and
arrives at the coast a short time before, or at the same moment with,
the shock, and produces the hollow sounds that are the harbingers
of ruin; then follows a continuous succession of sounds, like the
rolling of distant thunder, formed first, by the noise propagated in
undulations through the water of the sea, which travels at the rate
of 4700 feet in a second, and, lastly, by that passing through the
air, which only takes place when the origin of the earthquake is a
submarine explosion, and travels with the velocity of 1123 feet in
a second. The rolling sounds precede the arrival of the great
oceanic wave on the coasts, and are continued after the terrific catastrophe when the eruption is extensive.'
When there is a succession of shocks all the phenomena are repeated. Sounds sometimes occur when there is no earthquake: they
were heard on the plains of the Apure, in Venezuela, at the moment
the volcano in St. Vincent's, 700 miles off, discharged a stream of
lava. The bellowings of Guanaxuato afford a singular instance:
these subterraneous noises have been heard for a month uninter.
ruptedly when there was no earthquake felt on the table-land of
Mexico, nor in the rich silver-mines 1600 feet below its surface.
The velocity of the great oceanic wave varies as the square root
of the depth; it consequently has a rapid progress through deep
water, and less when it comes to soundings. That raised during the
earthquake at Lisbon travelled to Barbadoes at the rate of 7-8 miles
in a minute, and to Portsmouth at the rate of a little more than two
miles in a minute. The velocity of the shock varies with the elasticity of the strata it passes through. The undulations of the earth
are subject to the same laws as those of light and sound; hence,
when the shock or earth-wave passes through strata of different
elasticity, it will partly be reflected, and a wave will be sent back,
producing a shock in a contrary direction, and partly refracted, or
its course changed so that shocks will occur both upwards and downwards, to the right or to the left of the original line of transit.
Hence most damage is done at the junction of deep alluvial plains
Thus when an earthquake begins under the ocean, it occasions five distinct series of waves or undulations, all of which are subject to the same
laws of motion, namely, the earth-wave, the water-wave, and three other
series of waves arising from the passage of the sound, of the explosion
through the air, the earth, the water. For the laws of Sound, see Connexion of the Physical Sciences, 8th edition. [Also, "Hand-Books of
Natural Philosophy," by Dionysius Lardner, in which all that relates to the
subject is very clearly explained.]