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from the offing: the height of these waves has never been accurately mea- 
sured; that of the Lisbon earthquake, at Cadiz, was said to have been 60 feet, 
and about 18 feet at Funchal in Madeira. 
These waves have been repeatedly observed, and from a remote antiquity, 
as by Thucydides; and in South America some of the shores, or even lands 
comparatively remote from the shore, still present traces of their violent pas- 
sage in times very remote, with the same circumstances as at the present day. 
They are described as rolling in one long unbroken ridge of water with 
a steep impending front, and not breaking until after they had rolled some 
distance inland, overwhelming and sweeping away everything in their rapid 
and impetuous course. Darwin says, “It is remarkable that while Taleahuano 
and Callao near Lima, both situated at the head of large shallow bays, have 
suffered severely during every earthquake from great waves, Valparaiso, 
seated close to the edge of profoundly deep water, has never been over- 
whelmed, though so often shaken by the severest shocks.” 
A good account of the great earthquake of Talcahuano and of the great 
_ sea-waves thereof, will be found in the London Geogr. Journ. vol. vi. p. 319; 
the inundation of the sea, the author says, was similar to that recorded 
by Thucydides (iii. 89). 
The incoming of the great sea-wave, if in water of moderate depth and ona 
sloping beach, is immediately preceded by a slight recession of the water at 
the shore. 
The first great wave is often succeeded at intervals by others growing less 
and less; in the Lisbon earthquake there were eighteen such waves on the 
shore at Tangier. 
_ I must again refer to my ‘ Dynamics of Earthquakes,’ (Trans. Roy. Irish 
Acad.) for discussion of the theory of these great sea-waves. I have endea- 
voured to show that they are produced by the actual disturbance of the sea- 
bottom, directly over the point of original impulse ; that a wave is here gene- 
᾿ rated like that produced by dropping a stone into a pond, and is transmitted 
| in constantly enlarging circles, or at least in closed curved figures. This wave 
translates itself outward in all directions, with a speed dependent upon the 
_ depth of the water over which it is passing, and at last reaches the shore, or 
_ perhaps many distant shores, its line of motion being widely different at 
_ those points, perhaps, from that in which the shock was felt there. Its di- 
_ mensions depend upon the force of the original impulse and the extent of 
 sea-bottom simultaneously exposed to it, and the depth of the water and its 
force, time of breaking, &c. upon the form of the shores, depth of water close 
inland, and so forth. The view I have taken accounts for all the facts that 
have been recorded, satisfactorily to the demands of the science of tidal and 
- fluid wave motion; but I am thus brief upon this part of the subject because 
_ there are extremely few facts as to the dimensions, direction of motion, 
__ time of arrival, or other circumstances of great sea-waves that.have as yet 
been observed at all with accuracy. 
Navigators have often remarked and been placed in peril by, a peculiar sort 
| of inshore waves called “rollers” coming upon them suddenly and most unex- 
| pectedly: I just notice these here, as it remains as yet uncertain whether these - 
~ are nodal waves produced by the junction of several smaller waves far out on 
_ the ocean surface after storms, or be in some way connected with our subject. 
__ It seems probable that in the great ocean such vast nodal waves or rollers 
are frequently produced and propagated to great distances from the regions 
| of storm where they originate, and may simulate many of the phenomena of 
| earthquake great sea-waves. 
+ A good account of these will be found in Captain Owen’s narrative of the 
ON THE FACTS OF EARTHQUAKE PHA NOMENA. 47 
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