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Sect. VII.] 



EARTHQUAKE PHENOMENA 





01 



be made as to the height of this fluid ridge will afford 



of the height of the 



earta-wave or 



nide indications 



shock. 



Earthquakes, whether at sea or on land, seem to be 



only accompanied with subterranean noises when strata 

 are fractured or masses of matter blown away at volcanic 



Where such is not the case, the two preceding 

 are'the only waves to be expected from an earthquake of 

 inland origin ; but when fracture occurs, then at the 

 moment of the shock, or very slightly before or after it, 



wave, through the earth, ; 



and at an interval longer or shorter after this, 4th, the 



origms 



Sound 



Sound 



Again, when the origin of the earthquake i«! under the 

 ea (and such seems to bo the case with most great earth- 



^'■il 



quakes), 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 undulation of the 

 bottom of the sea gets into shallow water, and forces up a 

 ridge of water directly above itself, which it brings in to 



shore, and which seems to be the cause of that slight dis- 

 turbance of the margin of the sea often noticed as oc- 

 curring at the moment of the shock being felt ; 3. The 

 sound-wave through the earth (as in the former ease) ; 

 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 original impulse is not a 



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

 impulse extending along a considerable line of operation, 

 passing aAvay from the observer : the sound-waves will be 

 rumbling noises, and may be confounded in each medium 



k3 



