href="@public@vhost@g@gutenberg@html@files@33483@[email protected]#Footnote_B_2" class="fnanchor pginternal" tag="{http://www.w3.org/1999/xhtml}a">[B] This was rendered somewhat more definite by Gay Lussac, who, in an able paper "On the Chemical Theories of Volcanoes," in the twenty-second volume of the "Annales de Chémie," in 1823, says: "En un mot, les tremblements de terre ne sont que la propagation d'une commotion à travers la masse de la terre, tellement indépendante des cavités souterraines qu'elle s'entendrait, d'autant plus loin que la terre serait plus homogène."

These suggestions of Young and of Gay Lussac, as may be seen, only refer to the movement in the more or less solid crust of the earth. But two, if not three, other great movements were long known to frequently accompany earthquake shocks—the recession of the sea from the shore just about the moment of shock—the terrible sounds or subterraneous growlings which sometimes preceded, sometimes accompanied, and sometimes followed the shock—and the great sea-wave which rolls in-shore more or less long after it, remained still unknown as to their nature. They had been recognised only as concomitant but unconnected phenomena—the more inexplicable, because sometimes present, sometimes absent, and wholly without any known mutual bearing or community of cause.

On the 9th February, 1846, I communicated to the Royal Irish Academy my Paper, "On the Dynamics of Earthquakes," printed in Vol. XXI., Part I., of the Transactions of that Academy, and published the same year in which it was my good fortune to have been able to colligate the observed facts, and bringing them together under the light of the known laws of production and propagation of vibratory waves in elastic, solid, liquid and gaseous bodies, and of the production and propagation of liquid waves of translation in water varying in depth, to prove that all the phenomena of earthquake shocks could be accounted for by a single impulse given at a single centre. The definition given by me in that Paper is that an earthquake is "The transit of a wave or waves of elastic compression in any direction, from vertically upwards to horizontally, in any azimuth, through the crust and surface of the earth, from any centre of impulse or from more than one, and which may be attended with sound and tidal waves dependent upon the impulse and upon circumstances of position as to sea and land."

Thus, for example, if the impulse (whatever may be its cause) be delivered somewhere beneath the bed of the sea, all four classes of earthquake waves may reach an observer on shore in succession. The elastic wave of shock passing through the earth generally reaches him first: its velocity of propagation depending upon the specific elasticity and the degree of continuity of the rocky or the incoherent formations or materials through which it passes.

Under conditions pointed out by me, this elastic wave may cause an aqueous wave, producing recession of the sea, just as it reaches the margin of sea and land.

If the impulse be attended by fractures of the earth's crust, or other sufficient causes for the impulse to be communicated to the air directly or through the intervening sea, ordinary sound-waves will reach the observer through the air, propagated at the rate of 1,140 feet per second, or thereabouts; and may also reach him before or with or soon after the shock itself, through the solid material of the earth; and lastly, if the impulse be sufficient to disturb the sea-bottom above the centre of impulse, or otherwise to generate an aqueous wave of translation, that reaches the observer last, rolling in-shore as the terrible "great sea-wave," which has ended so many of the great earthquakes, its dimensions and its rate of propagation depending upon the magnitude of the originating impulse and upon the variable depth of the water. It is not my purpose, nor would it be possible within my limits here, to give any complete account of the matter contained in that Paper, which, in the words of the President of the Academy upon a later occasion, "fixed upon an immutable basis the true theory of Earthquakes."[C] I should state, however, that in it I proved the fallacy of the notion of vorticose shocks, which had been held from the days of Aristotle, and showed that the effects (such as the twisting on their bases of the Calabrian Obelisks) which had been supposed due to such, were but resolved motions, due to the transit rectilinearly of the shock.

This removed one apparent stumbling block to the true theory.

Incidentally also it was shown that from the observed elements of the movement of the elastic wave of shock at certain points—by suitable instruments—the position and depth of the focus, or centre of impulse, might be inferred.

In the same volume ("Transactions of the Royal I. Academy," XXI.) I gave account, with a design to scale, for the first self-registering and recording seismometer ever, to my knowledge, proposed. In some respects in principle it resembles that of Professor Palmieri, of which he has made such extended use at the Vesuvian Observatory, though it differs much from the latter in detail. In June, 1847, Mr. Hopkins, of Cambridge, read his Report, "On the Geological Theories of Elevation and Earthquakes," to the British Association—requested by that body the year before—and printed in its Reports for that year.

The chief features of this document are a digest of Mr. Hopkins's previously published "Mathematical Papers" on the formations of fissures, etc., by elevations and depressions, and those on the thickness of the earth's crust, based on precession, etc., which he discusses in some relations to volcanic action.

This extends to forty-one pages, the remaining eighteen pages of the Report being devoted to "Vibratory Motions of the Earth's Crust produced by Subterranean Forces—Earthquakes."

The latter consists mainly of a résumé of the acknowledged laws, as delivered principally by Poisson, of formation and propagation of elastic waves and of liquid waves, by Webers, S. Russel and others—the original matter in this Report is small—and as respects the latter portion consists mainly in some problems for finding analytically the position or depth of the centre of disturbance when certain elements of the wave of shock are given, or have been supposed registered by seismometric instruments, such as that described by myself, and above referred to.[D] At the time my original Paper "On the Dynamics of Earthquakes" was published, there was little or no experimental knowledge as to the actual velocity of transit of waves—analogous to those of sound, but of greater amplitude—through elastic solids. The velocity as deduced from theory, the solid being assumed quite homogeneous and continuous, was very great, and might be taken for some of the harder and denser rock formations at 11,000 or 12,000 feet per second. That these enormous velocities of wave transit would be something near those of actual earthquake shock seemed probable to me, and was so accepted by Hopkins.

Thus, he says (Report, p. 88): "The velocity of the sea-wave, for any probable depth of the sea, will be so small as compared with that of the vibratory wave, that we may consider the time of the arrival of the latter at the place of