200 



EABTHQUAKE FHEXOMENA. 



[Sect. VII. 



neously felt on tLe earth's surface. These may be called 



Coseismal lines. 



An erroneous notion, of the diracKsions of the great earth 



wave must not be formed from its being called an undula- 

 tion — its velocity of translation appears to be frequently as 

 much as thirty miles per minute, and the wave or shock 

 moving at this rate often takes ten or twenty seconds to 



pass a given point ; hence its length or amplitude is often 

 several miles. 



During the passage of the great earth wave or main 

 undulation, a continuous violent tremor or short quick un- 

 dulation (like a short chopping sea) is often felt. This 

 arises from secondary elastic waves, upon the surface of 

 the great earth wave, like the small curling or capillary 



waves on the 



surface 



of the ocean 



swell, but v\ 



hlch 



within its mass are analogous to the dispersion of light. 

 The physical cause of these is not here in point; but 

 it is very desirable that the interval in time between 

 these minor oscillations should be observed by a seconds 



watch. 



When the roll or undulation of the earth wave, 

 coming from inland, reaches the shores of the sea (unless 

 these be precipitous, with deep water), it may lift the 

 water of the sea up, and carry it along on its back, as it 

 were, as it goes out into deep water; for the rate of 

 transit of the shock is so immense that the elongated heap 



subside laterally. This 



of w 



ater lifted up has not time to 



/: 



comparatively sinall, and nearly the same as that of the 

 earth-wave, when close to the shore on a sloping beach ; 

 and where the water is still, any observations that can 



4 



t: 



1! 



