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218 



EARTHQUAKE PHENOMENA. 



[Sect. VII. 



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The duration of the sound from first to last, through 

 either medium, accompanying each shock, is important. 



Observations on the ffreat sea-wave should embrace, 

 for each wave, its height, its amplitude or length, its 

 velocity and direction of translation. The height to be 

 taken above the plane of repose of the fluid, and referred 

 to half-tide level. These waves, when on their grandest 

 scale, defy any methods of :;imeasurement (unless by 

 soundings where they break before coming to the beach), 

 but chance observations of their results, such as the 

 height to which they have reached on mural feces of 

 rock, or on such buildings, &c. as may withstand them, 

 or eve-si^ht observations made at the moment of transit 

 of the crest of the wave, cutting distant objects; but 

 when of a manageable si^e, the height of the crest may 

 be pretty closely obtained by the traces on wharfs, 

 buildings, &c., or on posts or piles driven into the littoral 

 bottom. This may be taken from any convenient fixed 

 points of level, and all ultimately referred to half-tide 

 level as the datum for all earthquake observations as to 

 level. 



The sextant may be occasionally used to get the eleva- 

 tion of the crest of the passing wave, several observers 



making a simultaneous observation of an expected wave 



The velocity of the wave may be got by noticing from a 



suitable position, by a seconds watch, the time of its 

 transit inwards between two distant points which should 

 have an interval of water whose depth is or may be known. 

 Islands off the land are advantageous posts for this 

 purpose. 



The length of the wave (while entire} should be sought 



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