earthquake, which took place on the 31st of August, 1886, are described
in great detail by Captain Clarence E. Dutton, of the U.S. Ordnance
Corps.[5] The conclusions arrived at are;--that as regards the depth of
the focal point, this is estimated at twelve miles, with a probable
error of less than two miles; while, as regards the rate of travel of
the earthquake-wave, the estimate is (in one case) about 3.236 miles per
second; and in another about 3.226 miles per second.

On the other hand, in the case of the earthquake of the Riviera, which
took place on the 23rd of February, 1887, at 5.30 a.m. (local time), the
vibrations of which appear to have extended across the Atlantic, and to
have sensibly affected the seismograph in the Government Signal Office
at Washington, the rate of travel was calculated at about 500 miles per
hour, less than one-half that determined in the case of Charleston; but
Captain Dutton claims, and probably with justice, that the results
obtained in the latter case are far more reliable than any hitherto
arrived at.

(_d._) _Oceanic Waves._--When the originating impact takes place under
the bed of the ocean--either by a sudden up-thrust of the crust to the
extent, let us suppose, of two or three feet, or by an explosion from a
submarine volcano--a double wave is formed, one travelling through the
crust, the other through the ocean; and as the rate of velocity of the
former is greatly in excess of that of the latter, the results on their
reaching the land are often disastrous in the extreme. It is the
ocean-wave, however, which is the more important, and calls for special
consideration. If the impact takes place in very deep water, the whole
mass of the water is raised in the form of a low dome, sloping equally
away in all directions; and it commences to travel outwards as a wave
with an advancing crest until it approaches the coast and enters shallow
water. The wave then increases in height, and the water in front is
drawn in and relatively lowered; so that on reaching a coast with a
shelving shore the form of the surface consists of a trough in front
followed by an advancing crest. These effects may be observed on a small
scale in the case of a steamship advancing up a river, or into a harbour
with a narrow channel, but are inappreciable in deep water, or along a
precipitous open coast.

(_e._) _The Earthquake of Lisbon, 1755._--The disastrous results of a
submarine earthquake upon the coast h