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pression though the solid crust of the earth, produced at any 

 distant points, by any original sufficient impulse, such as the 

 sudden bending, by elevation or depression, or the rupture of 

 a portion of the earth's crust. 



From this single principle, viz., that the true earthquake- 

 shock consists simply in the transit through the solid crust of 

 the earth of a wave of elastic compression, which the author 

 believes to be now, by him, for the first time, enunciated, 

 he proceeds to develope and account for, in detail, all the 

 more important recorded phenomena of earthquakes, as well 

 as many of the more perplexing secondary phenomena. 



The original impulse, or origin of an earthquake, may be 

 either under the sea, or on land, at a distance from the sea. 

 In the former case, at the moment of originating the impulse, 

 whether by bending or fracture, several distinct sets of waves 

 set out from the same points, and at the same moment of 

 time, but they move with very different velocities. 



The wave of elastic compression, or great earth-wave of 

 the author, makes its transit through the solid crust, outwards, 

 in all directions, from the point or points of impulse, and 

 moving at a speed proportionate to the specific elasticity and 

 density of the formations through which it passes. This, 

 the author shews, may be as much as 11,000 feet per second. 

 This wave constitutes a real undulation of the surface through 

 which it is passing, and may be also (if there is fracture at 

 the origin) heard as a sound-wave in the solid, moving at 

 the same rate. A sound-wave also travels through the water 

 of the sea, and, moving more slowly than in the solid, is heard 

 upon land after the shock has passed. Lastly, a rolling wave 

 of translation, or great sea-wave of the author, is formed by 

 the movement of the bottom, directly above the originating 

 disturbance. This sea-wave, though setting out at the same 

 moment as the shock or earth-wave, is rapidly outstripped by 

 the latter — because its motion is dependent upon its own form 

 and magnitude, and upon the depth of the sea upon and 



