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through which it moves. The great sea-wave, therefore, 
comes to land long after the shock has passed, and may be 
followed by several in succession, into which the original 
great sea-wave has broken, where the form of the soundings 
in-shore are suitable. 
The apparent recession of the sea just at the moment of 
the earthquake-shock reaching the land, the author shows, is 
to be accounted for by a small undulation of the sea, carried, 
as it were, upon the back of the earth-wave, and moved along 
at its speed, and which he has called the “ forced sea-wave.” 
Such are the usual train of circumstances when the centre of 
disturbance is under the sea, accompanied, in addition, by a 
sound-wave through the air, when rupture of the crust has 
occurred. 
When the centre of disturbance is far inland, the “ great 
earth-wave” and sound-wave through the solid, with the 
sound-wave through the air, and the ‘‘ forced-wave” upon the 
shore, are the only ones that can occur. 
Dr. Apjohn observed, that Mr. Mallet appeared to him not 
only to state very correctly how the onward motion of the 
earth’s crust, produced by an earthquake, might cause the 
upper stones of a pillar of masonry to be deranged from their 
position in the line of such motion, but also to have sug- 
gested, for the first time, the true cause of their partial rota- 
tion, or displacement in azimuth. In endeavouring, however, 
to remove the obvious objection to this explanation, viz., that 
the returning movement should restore such disturbed masses 
to their original position, he (Dr. Apjohn) could not but think 
that Mr. Mallet had introduced speculations as to the mecha- 
nism of terrestrial wave-motion, which, though very ingenious, 
appeared somewhat far-fetched and obscure, and, unless he 
(Dr. Apjohn) was much mistaken, certainly not necessary to the 
solution of the difficulty in question. In fact, it is impossible 
that the displacement produced by the forward motion could 
be undone by the returning stroke, unless upon a hypothesis 
