1835.1 CAUSE OF GREAT WAVE. .-^ng 



of the sea are said to have been greatly agitated. The distuib- 

 ance seems generally, as in the case of Concepcion, to have been 

 of two kinds : first, at the instant of the shock, the water swells 

 high up on the beach with a gentle motion, and then as quietly 

 retreats ; secondly, some time afterwards, the whole body of the 

 sea retires from the coast, and then returns in waves of over- 

 whelming force. The first movement seems to be an imme- 

 diate consequence of the earthquake affecting differently a fluid 

 and a solid, so that their respective levels are slightly de- 

 ranged : but the second case is a far more important phenome- 

 non. During most earthquakes, and especially during those on 

 the west coast of America, it is certain that the first great move- 

 ment of the waters has been a retirement. Some authors have 

 attempted to explain this, by supposing that the water retains its 

 level, whilst the land oscillates upwards ; but surely the water 

 close to the land, even on a rather steep coast, would partake of 

 the motion of the bottom: moreover, as urged by Mr. Lyell, 

 similar movements of the sea have occurred at islands far distant 

 from the chief line of disturbance, as was the case with Juan 

 Fernandez during this earthquake, and with Madeira during the 

 famous Lisbon shock. I suspect (but the subject is a very ob- 

 scure one) that a wave, however produced, first draws the water 

 from the shore, on which it is advancing to break : I have ob- 

 served that this happens with the little waves from the paddles 

 of a steam-boat. It is remarkable that whilst Tulcahuano and 

 Callao (near Lima), both situated at the head of large shallow 

 bays, have suffered during every severe earthquake from great 

 waves, Valparaiso, seated close to the edge of profoundly deep 

 water, has never been overwhelmed, though so often shaken by 

 the severest shocks. From the great wave not immediately fol« 

 lowing the earthquake, but sometimes after the interval of even 

 half an hour, and from distant islands being affected similarly 

 with the coasts near the focus of the disturbance, it appears that 

 the wave first rises in the offing ; and as this is of general occur- 

 rence, the cause must be general : I suspect we must look to the 

 line, where the less disturbed waters of the deep ocean join the 

 water nearer the coast, v/hich has partaken of the movements of 

 the land, as the place where the great wave is first generated ; it 

 would also appear that the wave is larger or smaller, according 



