2 



09 



EARTHQUAKE PHENOMENA. 



[Sect. VII 



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 lar.d. This is a wave of 

 translation : a heap of sea-water is thrown up at or over 

 the origin of the earthquake by tiie disturbance of the sea- 

 bottom, and begins to move off in waves like the circles on 

 a pond into which a pebble is dropped : and its phenomena 

 depend upon hi ws different from any of the other (elastic) 

 w^a\c3 of earthquakes. 



The original altitude (above the plane of repose of the 

 fluid) and volume of this wave depend upon the suddenness 

 and extent of the originating disturbance, and upon the 

 depth of water at its origin. Its velocity of translation 

 on the surface of the sea varies with the depth of the sea 

 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 v/ater 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 advanc- 

 ing slope becomes steeper, and when the depth of water 

 becomes less than the altitude of the wave, it topples over, 



gre 



Sometimes, how- 



ever, its volume, height, and velocity, are so great that it 

 comes ashore bodily and breaks far mland. The direction 

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

 necessarily infer that in which the 



oriedn mav be 



; as 



this wave may change its direction of motion greatly, or 

 become broken up into several minor v/aves in passing over 

 water varying much and suddenly in depth* or in following 

 the lines of a liighly-indented or island-girt shore. 



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