Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

378 CONCEPCION. March, 1835. disturbance, some little way distant. If the waves sent oflF from the paddles of a steam-vessel be watched breaking on the sloping shore of a still river, the water wUl 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 suifered 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- * I 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.