166 The East Anglian Earthquake.
physical connection. Had such effects been produced, we
might fairly have expected more numerous records from
the various towns and villages situated all along the junction
of the Tertiaries with the Chalk both north and south of the
London Basin.
Marginal Vibrations. — When a large tract of country is
shaken by an earthquake, the vibrations tend to make them-
selves felt with special distinctness along free margins, such
as coast-lines, river-valleys, and lines of outcrop, because in
these cases there is no resistance offered in one direction to
the vibrating particles in their outward movements. The
particles along such margins are in fact somewhat similarly
situated to the last of a row of marbles placed in contact with
one another, a blow delivered at one end of the series causing
the last marble in the row to move outwards. The perception
of a seismic disturbance at stations situated favourably with
respect to the foregoing conditions may for brevity be ascribed
to "marginal vibrations," although in the case of sea-coasts
and river-valleys the effects may be in part due to reflexion
and interference at the bounding surface of land and water,
i. e., to junctional vibrations. But although it may not be
possible to discriminate precisely between these two effects at
any particular station, it may be concluded generally that
the disturbance is more likely to be felt at such marginal
situations. This conclusion is in accordance with the laws of
earthquake-motion, as will appear from the following con-
siderations :—In spreading outwards from the centrum, the
vibrations have to pass through the solid earth, the particles
of which, owing to their constrained position, cannot oscillate
with any degree of freedom till the surface-layer is reached.
The motion is thus chiefly superficial, the maximum vertical
displacement taking place at the epicentrum, where the greatest
destruction occurs.69 As the distance from the epicentrum
increases the motion is chiefly horizontal, so that the con-
dition of horizontal restraint here prevents the particles from
69 Milne has found that at the bottom of a pit ten feet deep the motion
during an earthquake of sufficient intensity to crack buildings was only
one-fortieth of that at the surface.—' Nature,' July 16th, 1885, p. 201.