the eruptions of volcanoes; they cause a quivering of the earth, which
may be felt for a great distance from the immediate seat of the
disturbance.

When by any of the sudden movements which have been above described a
jar is applied to the rocks, the wave flies through the more or less
elastic mass until the energy involved in it is exhausted. This may
not be brought about until the motion has travelled for the distance
of hundreds of miles. In the great earthquake of 1755, known as the
Lisbon shock, the records make it seem probable that the movement was
felt over one eighth part of the earth's surface. Such great
disturbances probably bring about a motion of the rocks near the point
of origin, which may be expressed in oscillations having an amplitude
of one to two feet; but in the greater number of earthquakes the
maximum swing probably does not exceed the tenth of that amount. Very
sensible shaking, even such as may produce considerable damage to
buildings, are caused by shocks in which the earth vibrates with less
than an inch of swing.

When a shock originates, the wave in the rocks due to the compression
which the blow inflicts runs at a speed varying with the elasticity of
the substance, but at the rate of about fifteen hundred feet a second.
The movements of this wave are at right angles to the seat of the
originating disturbance, so that the shock may come to the surface in
a line forming any angle between the vertical and the nearly
horizontal. Where, as in a volcanic eruption, the shock originates
with an explosion, these waves go off in circles. Where, however, as
is generally the case, the shock originates in a fault plane, which
may have a length and depth of many miles, the movement has an
elliptical form.

If the earthquake wave ran through a uniform and highly elastic
substance, such as glass, it would move everywhere with equal speed,
and, in the case of the greater disturbances, the motion might be felt
over the whole surface of the earth. But as the motion takes place
through rocks of varying elasticity, the rate at which it journeys is
very irregular. Moving through materials of one density, and with a
rate of vibration determined by those conditions, the impulse is with
difficulty communicated to strata which naturally vibrate at another
speed. In many cases, as where a shock passing through dense
crystalline strata encounters a mass of soft sandstone, the wave, in
place of going on, is ref