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Sect. VI L] 



EARTHQUAKE PHENOMENA 



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ofF their sources, or from sudden elevation of the land, and 

 as sudden depression. This demands careful observation. 

 Observations of the forced sea-wave^ whether produced 

 by the earth-wave going out to sea or coming in from it. 

 will be nearly the same. It is desirable to find its height 

 above the surface of repose referred to half-tide level, 

 and its length or amplitude ; but from the extreme rapidity 

 of its production and cessation, or conversion into small 

 oscillatory waves lapping on the beach, and its generally 

 sni^ll altitude, observations are extremely difficult — they 

 are only possible v> I;en the surface of the sea is perfectly 

 calm, and then must be left to the skill of the observer in 

 taking advantage of local circumstances, and of evidence 

 as to the visible circumstances of this wave, which occurs 

 at the instant the shock is felt. 



Observations of the vjaves of sound throu 

 the sea, or fresh water and the air, are indicated pretty 

 fully by the description of these waves already given. 



The sound-wave through the earth travels at the same 

 rate as the shock, or earth-wave ; it is in fact the shock 

 (or its fractures) heard. Notice if the sound is heard 



before, along with, or after the shock is felt. An observer 



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putting one ear in close contact with the earth, and closing 

 the other, will hear the sound-wave through the earth 

 separate from that through the air. So also an observer 

 immersed in the sea will hear the sound-wave through it 

 sometimes without any complication of that through the 

 earth. 



The character and loudness of flie sound through each 

 medium, and the places in an extensive district vfhere 

 each was heard loudest and faintest, should be noted. 





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