364 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



These two are good types of those great sea waves in which the 

 water recedes from the land in the first few minutes following the 

 earthquake. 



Major Button and others have suggested that the sea bottom 

 sinks, and the explanation we have given of how this takes place 

 seems satisfactory and free from objections. It is clear that if by 

 the throes of the earthquake a large body of lava is forced from 

 under the bed of the Andean trough, the bed might thereupon settle 

 from twenty to fifty feet over a large area.^ The great inrush of 

 the water following this subsidence would withdraw it from the land, 

 and as soon as the rushing currents met in the center of the trough, 

 they would raise the water into a high ridge, and its subsidence would 

 give the first great wave which rolled in upon the devastated shore. 

 With the first depression of the water over the ridge, another in- 

 rush would take place, again withdrawing the sea from the shore, 

 and another great wave would follow like the first, but of slightly 

 feebler intensity. And so the oscillations of the sea would con- 

 tinue for a day or two, till they became reduced by friction to in- 

 sensible magnitude. This explanation accords with all the known 

 facts, and the recognized laws of fluid motion. Assuming it to be 

 correct, the result is of interest as showing the efifect of friction in 

 destroying the motion of the sea, which has often been discussed in 

 connection with the problem of the tides. In this case, the length of 

 the wave of 1868 has been calculated to be about 100 geographical 

 miles ; and as the depth of the sea is between four and five miles, we 

 see that the wave length involved is from twenty to twenty-five times 

 the depth of the sea. 



At the close of this section we shall give another possible ex- 

 planation of waves of this kind, which begins with a recession of 

 the water from the shore ; but meanwhile we shall notice waves of 

 a different class, sometimes encountered, which begin by a sudden 

 rising of the water near shore. 



The great wave which overwhelmed Simoda, Japan, December 

 29, 1854, may be taken as a type of those which are characterized 

 by the sudden inrush of a great wave without any previous recession 



^The subsidence might be much greater if the area affected was propor- 

 tionally diminished. 



