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Ch. XXX.] 



EAETHQUAKE-WAVES IN THE SEA. 



151 



Madeira 



tlie sea-wave took 2i 



same 



'B 



time wliicli it required to reacli other places according to 

 their distance. We cannot^ therefore, explain the great 

 motion of the waters at Madeira, by a momentary upward 

 movement of the solid crust of the earth, for in that case the 

 rise of the beach would have occurred at the first period or 

 25 minutes after the Lisbon shock ; besides, it will be seen in 

 the sequel, page 153, that where the sea is deep near the shore, 

 and the beach very steep, as in Madeira, the land-wave can- 

 not cause a retreat of the sea. 



The following is another solution of the problem, which 

 has been offered : — Suppose a portion of the bed of the sea 

 to be suddenly upheaved ; the first effect will be to raise over 

 the elevated part a body of water, the momentum of Avhicli 

 will carry it much above the level it will afterwards assume, 

 causing a draught or receding of the water from the neigh- 

 bouring coasts, followed immediately by the return of the 

 displaced water, which will also be impelled by its momentum 

 much farther and higher on the coast than its former leveL"^ 



Mr. Darwin, when alluding to similar waves on the coast 

 of Chili, states his opinion, that ^ the whole phenomenon is 

 due to a common undulation in the water, proceeding from a 

 line or point of disturbance some little way distant. If the 

 waves, ^ he says, *^ sent off from the paddles of a steam-vessel 

 be watched breaking on the sloping shore of a still river, the 

 Vv-ater will be seen first to retire two or three feet, and then 

 to return in little breakers, precisely analogous to those con- 

 sequent on an earthquake.' He also adds, that "^the earth- 

 quake-wave occurs some time after the shock, the Avater at 

 first retiring both from the shores of the mainland and of 

 outlying islands, and then returning in mountainous breakers. 

 Their size is modified by the form of the neighbouring coast ; 

 for it is ascertained in South America, that places situated at 

 the head of shoaling baj^s have suffered most, whereas towns 



lik 



e 



raiso, seated close on the border of a profound 



^' Quarterly Eevlew, No. Ixxxvi. p. 459 



