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 earth. 



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 -hows that 

 1 iiiaJe to 



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jxii' 



Ch. XXX.] 



GREAT WAVE OF THE SEA. 



153 



because it ' depends upon a function of the elasticity of tlie 

 crust of tlie earth, whereas the velocity of the sea-wave 

 depends upon a function of the depth of the sea.' 



' Although the shock in its passage under the deep ocean 



;'ives no trace of its progress, it no sooner gets into soundings 



or shallow water, than it gives rise to another and smaller 

 wave of the sea. It carries, as it were, upon its back, this 

 lesser aqueous undulation ; a long narrow ridge of water, 

 which corresponds in form and velocity to itself, being pushed 

 up by the partial elevation of the bottom. It is this small 



com 



municates 



the earthquake-shock to ships at sea, as if they 

 had struck upon a rock. It breaks upon a coast at the same 



moment 



sometimes 



cause an apparent slight recession from the shore, followed 

 by its flowing up somewhat higher than the usual tide mark : 

 this will happen where the beach is very sloping, as is usual 

 where the sea is shallow, for then the velocity of the low flat 

 earth-wave is such, that it slips, as it were, from under the 

 undulation in the fluid above. It does this at the moment 

 of reaching the beach, which it elevates by a vertical height 

 equal to its own, and as instantly lets drop 

 former level.' 



again to its 



' While the shock propagated through the soKd earth has 

 thus travelled with extra rapidity to the land, the great sea- 

 wave has been following at a slower pace, though advancing 

 at the rate of several miles in a minute. It consists, in the 

 deep ocean, of a long low sv/ell of enormous volume, having an 

 equal slope before and behind, and that so gentle that it 

 might pass under a ship without being noticed. But when 

 it reaches the edge of soundings, its front slope, like that of 

 a tidal wave under similar circumstances, becomes short and 

 steep, while its rear slope is long and gentle. If there be 

 water of some depth close into shore, this great wave may 

 roll in long after the shock, and do little damage : but if 

 the shore be shelving, there will be first a retreat of the 

 water, and then the wave will break upon the beach and roll 

 in far upon the land.' ^ 



* Mallet, Proceed. Rov. Irish Acad. 184G. 



»* ■ -1. 



