e. At Loch Lomond, in
Scotland, for example, the water, without, the least apparent cause,
rose against its banks, and then subsided below its usual level. The
greatest perpendicular height of this swell was two feet four inches. It
is said that the movement of this earthquake was undulatory, and that it
travelled at the rate of twenty miles a minute, its velocity being
calculated by the intervals between the time when the first shock was
felt at Lisbon, and its time of occurrence at other distant places.[685]

_Great wave and retreat of the sea._--A great wave swept over the coast
of Spain, and is said to have been sixty feet high at Cadiz. At Tangier,
in Africa, it rose and fell eighteen times on the coast. At Funchal, in
Madeira, it rose full fifteen feet perpendicular above high-water mark,
although the tide, which ebbs and flows there seven feet, was then at
half-ebb. Besides entering the city, and committing great havoc, it
overflowed other seaports in the island. At Kinsale, in Ireland, a body
of water rushed into the harbor, whirled round several vessels, and
poured into the market-place.

It was before stated that the sea first retired at Lisbon; and this
retreat of the ocean from the shore, at the commencement of an
earthquake, and its subsequent return in a violent wave, is a common
occurrence. In order to account for the phenomenon, Michell imagined a
subsidence at the bottom of the sea, from the giving way of the roof of
some cavity in consequence of a vacuum produced by the condensation of
steam. Such condensation, he observes, might be the first effect of the
introduction of a large body of water into fissures and cavities already
filled with steam, before there has been sufficient time for the heat of
the incandescent lava to turn so large a supply of water into steam,
which being soon accomplished causes a greater explosion.

Another proposed explanation is, the sudden rise of the land, which
would cause the sea to abandon immediately the ancient line of coast;
and if the shore, after being thus heaved up, should fall again to its
original level, the ocean would return. This theory, however, will not
account for the facts observed during the Lisbon earthquake; for the
retreat preceded the wave, not only on the coast of Portugal, but also
at the island of Madeira, and several other places. If the upheaving of
the coast of Portugal had caused the retreat, the motion of the waters,
when propagated to Mad