d partially round, and removed sometimes nine inches from their
position without falling.

It has been suggested by Mr. Darwin that this kind of displacement may
be due to a vibratory rather than a whirling motion;[668] and more
lately Mr. Mallet, in the paper already cited, has offered a very
ingenious solution of the problem. He refers the twisting simply to an
elastic wave, which has moved the pedestal forwards and back again, by
an alternate horizontal motion within narrow limits, and he has
succeeded in showing that a rectilinear movement in the ground may have
sufficed to cause an incumbent body to turn partially round upon its
bed, provided a certain relation exist between the position of the
centre of gravity of the body and its centre of adherence.[669]

I shall now consider, in the first place, that class of physical changes
produced by the earthquake which are connected with alterations in the
relative level of the different parts of the land; and afterwards
describe those which are more immediately connected with the derangement
of the regular drainage of the country, and where the force of running
water co-operated with that of the earthquake.

_Difficulty of ascertaining changes of level._--In regard to alterations
of relative level, none of the accounts establish that they were on a
considerable scale; but it must always be remembered that, in proportion
to the area moved is the difficulty of proving that the general level
has undergone any change, unless the sea-coast happens to have
participated in the principal movement. Even then it is often impossible
to determine whether an elevation or depression even of several feet has
occurred, because there is nothing to attract notice in a band of
shingle and sand of unequal breadth above the level of the sea running
parallel to a coast; such bands generally marking the point reached by
the waves during spring tides, or the most violent tempests. The
scientific investigator has not sufficient topographical knowledge to
discover whether the extent of beach has diminished or increased; and he
who has the necessary local information, scarcely ever feels any
interest in ascertaining the amount of the rise or fall of the ground.
Add to this the great difficulty of making correct observations, in
consequence of the enormous waves which roll in upon a coast during an
earthquake, and efface every landmark near the shore.

_Subsidence of the quay at Messina._--It is