1835.) CAUSE OF GREAT WAVE. 809 
of the sea are said to have been greatly agitated. The disturb- 
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, which 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 
