These results have since received general confirmation by the careful determinations of the transit rates of actual earthquake waves, in the rocks of the Rhine Country and in Hungary, by Nöggerath and Schmidt respectively, and by those made since by myself in those of Southern Italy, to which 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 connect the phenomena admitted by reference to any general mechanical or physical causes. In 1850 my first "Report upon the Facts of Earthquakes," called for by the British Association in 1847, was read and published in the Reports of that body for that year. In this, for the first time, the many recorded phenomena of Earthquakes are classified, and the important division of the phenomena into primary and secondary effects of the shock was established. Several facts or phenomena, previously held as marvellous or inexplicable, were either, on sufficient grounds, rejected, or were, for the first time, shown susceptible of explanation. Amongst the more noticeable results were the pointing out that fissures and fractures of rock or of incoherent formations were but secondary effects, and, in the latter, were, in fact, generally of the nature of inceptive landslips. This last was not accepted, I believe, by geologists at the time; but the correctness of the views then propounded as to earth fissures—the nature of the spouting from them of water or mud—the appearances taken for smoke issuing from them, etc.—have since been fully confirmed, first, by my own observations upon the effects of the Great Neapolitan Earthquake of 1857, and more lately by those of Dr. Oldham upon the Earthquake of Cachar (India), where he was enabled to observe fissures of immense magnitude, the nature of the production of which he has well described and explained in the "Proceedings, Geological Society, London, 1872."

The relations between meteorological phenomena proper and Earthquakes have always been a subject of popular belief and superstition.

This was here carefully discussed, and with the result of disproving any connection, or, if any, but of an indirect nature. I also, to some extent, towards the end of this Report, discussed the question of the possible nature of the impulse itself which originates the shock; I showed that it must be of the nature of a blow, and ventured to offer conjecturally five possible causes of the impulse:

  1. Sudden fractures of rock, resulting from the steady and slow increase of elevatory pressure.
  2. Sudden evolution (under special conditions) of steam.
  3. Sudden condensation of steam, also under special conditions.
  4. Sudden dislocations in the rocky crust of the earth, through pressure acting in any direction.
  5. Occasionally through the recoil due to explosive effects at volcanic foci (p. 79-80).

The first and last of these I am, through subsequent light, disposed now to withdraw or greatly to modify.