wave particle or wave itself.

This applies also to the demonstration and diagram (taken from Hopkins)
given by Professor Phillips ("Vesuvius," pp. 258-259).

In December, 1857, occurred the great Neapolitan Earthquake, which
desolated a large portion of that kingdom; and an opportunity then arose
for practically applying to the problems of finding the directions of
earthquake shock at a given point through which it has passed, and
ultimately the position and depth of focus, other methods, which I had
seen, from soon after the date of publication of my original Paper
(1846), were easily practicable, and the details of which I had
gradually matured.

Bearing in mind that, in the case of the normal vibration in any elastic
solid of indefinite dimensions, the direction of motion in space of the
_wave particle_ coincides in the first semiphase of the wave, and at the
instant of its _maximum velocity_ with the right line joining the
particle and the focus or centre of disturbance, it follows that, in the
case of earthquakes, the normal vibration of the wave of shock is always
in a vertical plane passing through the focus and any point on the
earth's surface through which the shock passes (assuming for the present
no disturbing causes after the impulse has been given), and that at such
a point the movement of the wave particle in the first semiphase of the
wave is in the same direction or sense as that of translation; and at
the moment of maximum velocity the direction in space of the motion of
the wave particle is that of the right line joining the point through
which the wave has passed with the focus or centre of impulse.

If, therefore, we can determine the direction of motion of the wave
particle in the first semiphase, and its maximum velocity, we can
obtain, from any selected point, a line (that of emergence of the shock)
_somewhere in which_, if prolonged beneath the earth, the focus must
have been; and if we can obtain like results for two or more selected
points, we decide the position and the depth of the focus, which must be
in the intersection of the several lines of direction of the wave
particle motion at each point, when prolonged downwards.

Now, as I have said, it is the _vibration of the wave itself_, _i.e._,
the motion of the wave particle that does the mischief--_not_ the
transit of the wave from place to place on the surface; just as in the
analogous (but _not_ similar) case of a tidal wave of transl