ena of earthquake shocks could be accounted for by a single
impulse given at a single centre. The definition given by me in that
Paper is that an earthquake is "_The transit of a wave or waves of
elastic compression in any direction, from vertically upwards to
horizontally, in any azimuth, through the crust and surface of the
earth, from any centre of impulse or from more than one, and which may
be attended with sound and tidal waves dependent upon the impulse and
upon circumstances of position as to sea and land_."

Thus, for example, if the impulse (whatever may be its cause) be
delivered somewhere beneath the bed of the sea, all four classes of
earthquake waves may reach an observer on shore in succession. The
elastic wave of shock passing through the earth _generally_ reaches him
first: its velocity of propagation depending upon the specific
elasticity and the degree of continuity of the rocky or the incoherent
formations or materials through which it passes.

Under conditions pointed out by me, this elastic wave may cause an
aqueous wave, producing recession of the sea, just as it reaches the
margin of sea and land.

If the impulse be attended by fractures of the earth's crust, or other
sufficient causes for the impulse to be communicated to the air directly
or through the intervening sea, ordinary sound-waves will reach the
observer through the air, propagated at the rate of 1,140 feet per
second, or thereabouts; and may also reach him before or with or soon
after the shock itself, through the solid material of the earth; and
lastly, if the impulse be sufficient to disturb the sea-bottom above the
centre of impulse, or otherwise to generate an aqueous wave of
translation, that reaches the observer last, rolling in-shore as the
terrible "great sea-wave," which has ended so many of the great
earthquakes, its dimensions and its rate of propagation depending upon
the magnitude of the originating impulse and upon the variable depth of
the water. It is not my purpose, nor would it be possible within my
limits here, to give any complete account of the matter contained in
that Paper, which, in the words of the President of the Academy upon a
later occasion, "fixed upon an immutable basis the true theory of
Earthquakes."[C] I should state, however, that in it I proved the
fallacy of the notion of vorticose shocks, which had been held from the
days of Aristotle, and showed that the effects (such as the twisting on
their