h I shall again refer. In an elastic wave propagated from
a centre of impulse in an infinitely extended volume of a perfect gas,
normal vibrations are alone propagated--as is the case with sound in
air.

In the case of like movements propagated in elastic and perfectly
homogeneous and isotropic solids, the wave possesses both normal and
transversal vibrations, and is, in so far, analogous to the case of
light. Mr. Hopkins, in his Report above referred to, has based certain
speculations upon the assumed necessary co-existence of both orders of
vibration in actual earthquake shocks in the materials of which our
earthy crust is actually composed.

The existence of transversal vibration in those materials has not been
yet proved experimentally, though there is sufficient ground to preclude
our denying their probable existence.

That if they do exist they play but a very subordinate part in the
observable phenomena of actual Earthquake is highly probable. This is
the view, supported not only by observations of the effects of such
shocks in Nature, but by the theoretic consideration of the effects of
discontinuity of formations in planes or beds more or less transverse to
the wave path (or line joining the centre of impulse with the mean
centre of wave disturbance at any point of its transit). If we suppose,
for illustration sake, such an elastic wave transmitted perpendicularly
through a mass of glass plates, each indefinitely thin, and all in
absolute contact with each other, but without adhesion or friction, more
or less of the transversal vibration of the wave would be cut off and
lost at each transit from plate to plate, as the elastic compression
can, by the conditions, be transmitted only normally or by direct push
perpendicularly from plate to plate. This must take place in Nature, and
to a very great extent, and the consideration, with others, enabled me
generally to apply the normal wave motion of shock alone to my
investigation as to the depth of the centre of impulse of the great
Neapolitan Earthquake of 1857, an account of which was published in
1862, and to be presently further referred to.

Hitherto the multitudinous facts, or supposed facts, recorded in
numberless accounts of Earthquakes had remained almost wholly
unclassified, and so far as they had been discussed--in a very partial
manner, as incidental portions of geological treatises--with little
attempt to sift the fabulous from the real, or to connec