a is shallow, for then the velocity of the low flat earth-wave is
such, that it slips as it were, from under the undulation in the fluid
above. It does this at the moment of reaching the beach, which it
elevates by a vertical height equal to its own, and as instantly lets
drop again to its former level."

"While the shock propagated through the solid earth has thus travelled
with extra rapidity to the land, the great sea-wave has been following
at a slower pace, though advancing at the rate of several miles in a
minute. It consists, in the deep ocean, of a long low swell of enormous
volume, having an equal slope before and behind, and that so gentle that
it might pass under a ship without being noticed. But when it reaches
the edge of soundings, its front slope, like that of a tidal wave under
similar circumstances, becomes short and steep, while its rear slope is
long and gentle. If there be water of some depth close into shore, this
great wave may roll in long after the shock, and do little damage; but
if the shore be shelving, there will be first a retreat of the water,
and then the wave will break upon the beach and roll in far upon the
land."[690]

The various opinions which have been offered by Michell and later
writers, respecting the remote causes of earthquake shocks in the
interior of the earth, will more properly be discussed in the
thirty-second chapter.

_Chili_, 1751.--On the 24th of May, 1751, the ancient town of
Conception, otherwise called Penco, was totally destroyed by an
earthquake, and the sea rolled over it. (See plan of the bay, fig. 70,
p. 455.) The ancient port was rendered entirely useless, and the
inhabitants built another town about ten miles from the sea-coast, in
order to be beyond the reach of similar inundations. At the same time, a
colony recently settled on the sea-shore of Juan Fernandez was almost
entirely overwhelmed by a wave which broke upon the shore.

It has been already stated, that in 1835, or eighty-four years after the
destruction of Penco, the same coast was overwhelmed by a similar flood
from the sea during an earthquake; and it is also known that twenty-one
years before (or in 1730), a like wave rolled over these fated shores,
in which many of the inhabitants perished. A series of similar
catastrophes has also been tracked back as far as the year 1590,[691]
beyond which we have no memorials save those of oral tradition. Molina,
who has recorded the customs and legends