378 



CONCEPCION. 



March, 1835. 



disturbance, some little way distant. If the waves sent oif 

 from the paddles of a steam-vessel be watched breaking on 

 the sloping shore of a still river, the water will be seen first 

 to retire two or three feet, and then to return in little breakers, 

 precisely analogous to those consequent on an earthquake. 

 From the oblique direction in which the waves are sent off 

 from the paddles, the vessel has proceeded a long way ahead, 

 before the undulation reaches the shore ; and hence it is at 

 once manifest, that this movement bears no relation to the 

 actual displacement of the fluid from the bulk of the vessel. 

 Indeed, it seems a general circumstance, that in all cases 

 where the equilibrium of an undulation is thus destroyed, 

 the water is drawn from the resisting surface to form the 

 advancing breaker.* Considering then a wave produced by 

 an earthquake as an ordinary undulation proceeding from 

 some point or line in the offing, we can see the cause, first 

 of its occurrence some time after the shock ; secondly, of 

 its affecting the shores of the mainland and of outlying islets 

 in a uniform manner — namely, the water retiring first, and 

 then returning in a mountainous breaker ; and lastly, of its 

 size being modified (as appears to be the case) by the 

 form of the neighbouring coast. For instance Talcuhano 

 and Callao are situated at the head of great shoaling bays, 

 and they have always suff'ered from this phenomenon ; 

 whereas, the town of Valparaiso, which is seated close on the 

 border of a profound ocean, though shaken by the severest 

 earthquakes, has never been overwhelmed by one of these 

 terrific deluges. On this view, we have only to imagine, in 

 the case of Concepcion, a point of disturbance in the bottom 

 of the sea in a south-west direction, whence the wave was 

 seen to travel, and where the land was elevated to a greater 

 height than any other part, — and the whole phenomenon will 

 be explained. 



It is probable that near every coast, the chief line of dis- 



* 1 am indebted to Mr. Whewell for explaining to me the probable 

 movements on the shore, of an undulation of which the equilibrium has 

 been destroyed. 



