ation
running up an estuary and passing a ship anchored there, it is not the
transit up the channel, but the wave form itself--_i.e._, the motion of
the wave particles--that lifts the ship, sends her a little way higher
up channel, drops her to her former level, and sends her down channel
again to the spot she lay in just before the arrival of the wave.

Everything, therefore, that has been permanently disturbed by an
earthquake shock has been thus moved in the direction and with the
maximum velocity impressed upon it by the wave particle in the first
semiphase of the wave; and thus almost everything that has been so
disturbed may, by the application of established dynamical principles,
be made to give us more or less information as to the velocity of the
wave particle (or as we, for shortness, say, the velocity of shock), the
direction of its normal vibration, and the position and depth beneath
the earth's surface, from which came the generating impulse. We thus
arrive at these as simply and as surely as we can infer from the
position taken by a billiard ball, on which certain forces are known to
have acted, the forces themselves and their direction; or, from a broken
beam, the pressure or the blow which fractured it.

It is obvious, then, that nearly every object disturbed, dislocated,
fractured or overthrown by an earthquake shock is a sort of natural
seismometer, and the best and surest of all seismometers, if we only
make a judicious choice of the objects which being found after such a
shock, we shall employ for our purpose. This was the principle which I
proposed to the Royal Society at once to apply to the effects of the
then quite recent great Neapolitan Earthquake of 1857, and which,
through the liberality and aid of that body, I was enabled to employ
with the result I had pretty confidently anticipated, namely, the
ascertainment of the approximate depth of the focus.

_Every_ shock-disturbed object in an earthquake-shaken country is
capable of giving _some_ information as to the shock that acted upon it;
but it needs a careful choice, and some mechanical [Greek: nous], to
select _proper_ and the best objects, so as to avoid the needless
perplexity of disturbing forces _not_ proper to the shock, or other
complications.

When properly chosen, these natural seismometers, or evidences fitted
for observation after the shock, are of two great classes, by which the
conditions of the earthquake motion are discover