166 SEE— FURTHER RESEARCHES ON [April 24, 



§ 5. Important Criterion for the Nature of the Movement Be- 

 neath the Earth's Crust furnished by Seismic Sea Waves. — In the 

 paper on the " Cause of Earthquakes " we divided seismic sea 

 waves into two general classes : the first, due to the sinking of the 

 sea bottom, and characterized by a withdrawal of the water after 

 the earthquake, to be followed later by the return of a great wave; 

 the second, due to the uplift of the bottom, and characterized by 

 the sudden rise of the sea without any previous withdrawal from 

 the shore. Both classes of these waves exist in our seas, but those 

 of the first class are the most dangerous and the most important. 

 Most of the great historical inundations by the sea have been due 

 to waves of the first class. The phenomena usually noted are : 

 first, a terrible earthquake; second, after a short interval, the sea 

 is noticed to be slowly draining away, laying bare the bottom, 

 where it ordinarily is deep enough to anchor ships ; third, after an 

 interval of an hour or so, the sea is seen to be returning as a 

 mighty wave, carrying everything before it, and thus washing the 

 ships inland and stranding them high and dry ; fourth, having once 

 swept the shore, the sea again withdraws and lays bare the harbor 

 as before, and after about the same interval again returns as a 

 second great wave. This periodic movement of the sea may be kept 

 up for quite a while, and sometimes quiet is not restored for a 

 day or two. 



Among the many well-known historical sea waves of the first 

 class which might be mentioned, we shall cite only a few typical 

 cases: As that which overwhelmed Helike in 373 B. C. (see the 

 paper on the " Temperature of the Earth," § 23, pp. 269-272, and 

 Addendum, pp. 291-298); the wave at Callao in 1746; the wave 

 following the Lisbon earthquake in 1755; the waves of Arica, 1868, 

 and Iquique, 1877; the wave on the Japanese coast in 1896. In all 

 these cases the water first withdrew from the shore; not suddenly, 

 but slowly, as in the draining away of a tide, though somewhat 

 more rapidly; this of course indicated that the sea bottom had 

 sunk, and the water was draining away to fill up the depression in 

 the level caused by the drop of the bottom. When the currents meet 

 at the center an elevation is produced by their mutual impact, and 



