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Sect, VII.] 



EARTHQUAKE PHENOMENA 



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for by a similar method ; a knowledge of Its length and 

 of the depth of water infers its height. 



The point approaching the coast at which the wave is 

 first observed to break, when capable of being accurately 

 found, gives the height of the wave, which is here equal 



to the depth of the soundings. Tliis breaking point and 



depth should always be anxiously tried for. Besides the 

 dimensions of the wave, observations should be made on 

 the interval of time after the great earth-wave, or shock, 

 before the great sea-wave comes in, reckoning from the 

 commencement of the shock ; how inany such waves in 

 succession ; what is the period between each ; what are 



their relative dimensions 



what changes 





re observable 



in the directions whence they come at the same point 

 of coast, and what are the several in-coming directions 

 at various points along a great stretch of coast (the 

 latter must be had usually from collected testimony) ; 

 what reflux from the beach before or after the cominfr 

 in of the wave ; after the wave has come in and l»roken, 

 what oscillatory waves are producedj their character and 

 dimensions ; is the level of the surface of the sea in 

 repose, the same before and after the subsidence of the 

 great sea-wave and its secondary or oscillatory waves ; 

 what the state of tide above or below half-tide level at 

 the moment of shock and of great sea-wave. 



As accurate a section as possible of the form of the 

 littoral bottom, beach, offing, and out to deep water, should 

 be got by soundhigs in the line of the coming in of the 

 wave, and laid do^ii on paper ; and, where possible, a 

 cruise should be made out to sea in the direction whence 

 the waves came, to look for pumice, dead fish, volcanio 



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