CAUSE OF GREAT WAVE. 55 



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 overwhelm- 

 ing force. The first moveinent seems to be an 

 immediate consequence of the earthquake affecting 

 differently a fluid and a solid, so that their respect- 

 ive levels are slightly deranged : but the second 

 case is a far more important phenomenon. During 

 most earthquakes, and especially during those on 

 the west coast of America, it is certain that the first 

 great movement of the waters has been a retire- 

 ment. 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 move- 

 ments of the sea have occurred at islands far distant 

 from the chief line of distui'bance, 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 obscure one) that 

 a wave, however produced, first draws the water 

 from the shore on which it is advancing to break : 

 I have observed that this happens with the little 

 waves from the paddles of a steamboat. It is re- 

 markable, that whilst Talcahuano 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 seve- 

 rest shocks. From the great wave not immediately 

 following the earthquake, but sometimes after the 

 interval of even half an hour, and from distant isl- 



