EARTHQUAKE PHENOMENA. 413 



hear, 3d, the Sound-wave through the earth; and at an interval longer 

 or shorter after this, 4th, the Sound-wave through the air. 



Again, when the origin of the earthquake is under the sea, (and 

 such seems to he the case with many great earthquakes,) we may 

 expect in the following order: 1. The great earth-wave or shock; 

 2. The forced sea-wave, which is formed as soon as the true shock 

 or coseismal undulation of the bottom of the sea gets into shallow 

 water, and forces up a ridge of water directly above itself, which 

 accompanies it to shore, and which seems to be the cause of that 

 slight disturbance of the margin of the sea often noticed as occurring 

 at the moment of the shock being felt; 3. The sound-wave through 

 the earth, (as in the former case;) 4. The sound-wave through the 

 sea, which arrives after that through the earth, but prior to, 5. The 

 sound-wave through the air. Where the originating force is not a 

 single impulse, but a quick succession of these, or a single impulse 

 extending along a considerable line of disturbance, passing away 

 from the observer, the sound-waves will be rumbling noises, and 

 may be confounded in each medium more or less; and where no 

 fractures or explosions occur, the sound-waves may be wholly 

 wanting. 



Lastly, and usually a considerable time after the shock, the great . 

 sea-wave rolls in to land. This is a ivave of translation ; a heap of 

 sea-water is thrown up at or over the origin of the earthquake by the 

 actual disturbance of the sea-bottom, or in the direction, and by the 

 emergence, of the eorth-wave beneath the sea at a large angle to the 

 horizon, and begins to move oft" in waves like the circles on a pond 

 into which a pebble is dropped; and its phenomena depend upon laws 

 different from any of the other (elastic) waves of earthquakes. 



The original altitude (above the plane of repose of the fluid) and 

 volume of this liquid wave depend upon the suddenness and extent 

 of the originating disturbance, and upon the depth of water above 

 its origin. Its velocity of translation on the surface of the sea varies 

 with the depth of the water at any given point, and its form and 

 dimensions depend upon this also, as well as upon the sort of sea- 

 room it has to move in. In deep-ocean water one of these waves 

 may be so long and low as to pass under a ship without being observed; 

 but as it approaches a sloping shore its advancing slope becomes 

 steeper, and when the depth of water becomes less than the altitude 

 of the wave, it topples over, and comes ashore as a great breaker. 

 Sometimes, however, its volume, height, and velocity, are so great 

 that it comes ashore bodily and breaks far inland. The direction 

 from which it arrives at any given point of land does not necessarily 

 infer that in which the origin may be; as this wave may change its 

 direction of motion greatly, or become broken up into several minor 

 waves in passing over water varying much and suddenly in depth, 

 or in following the lines of a highly-indented or island-girt shore. — 

 (See Airy on Tides, Encyc. Metrop. ; Russel, Report on Waves, Brit. 

 Ass., 1844; Bache, Great Sea Waves in Pacific, Amer. Jour. Science, 

 vol. xxi, 1856; Mallet, 4th Report, 1857-58; Darwin, Voyage of 

 the Beagle.) 



