162 DYNAMICAL GEOLOGY. 



is enormously great for water waves. The wave of 1854 

 traversed the Pacific, from Japan to San Francisco, a dis- 

 tance of 4,500 miles, in about twelve hours, or at a rate 

 of 370 miles an hour. The wave of 1868 ran across the 

 Pacific with even greater speed. The reason of their 

 great velocity is their enormous size. 



I. The size of the great sea wave is determined by the 

 principle that every wave runs its own length in the time 

 of one -oscillation. If a boat be lying on smooth water, 

 and a series of water waves passes under it, the boat will 

 be moved up and down once while the waves run the 

 length of one wave ; i. e., from trough to trough. Now, 

 the time of oscillation of the great sea waves of 1854 was 

 about thirty-three minutes. If, then, the waves run 370 

 miles in an hour (60 minutes), how much did they run in 

 33 minutes 60 : 33 : : 370 : 203. Therefore, these waves 

 were 203 miles from trough to trough. 



c. The mean depth of the ocean may be determined by 

 these waves. The principle on which this is done is as 

 follows : Every one has observed that waves coming in 

 from deep water on to a flat, shelving shore, at a certain 

 depth begin to drag bottom, and are impeded thereby ; 

 also, that the larger the wave, the deeper the water in 

 which it begins to drag. Now, in the case of these enor- 

 mous earthquake sea waves, the ocean itself is not deep 

 enough to prevent them from dragging bottom. As they 

 run over the sea their velocity is impeded everywhere, 

 but more or less according to the varying depth of the 

 ocean. Now, the normal or unimpeded velocity of a wave 

 may be accurately calculated, since it varies as the square 

 root of the wave-length (v oc VL)- Therefore, the 

 amount of retardation will give the depth of the ocean 

 over which it passes. The mean depth of the ocean 

 between Japan and San Francisco, as thus determined, 

 is 12,000 feet ; between Arica and Hawaii it is 18,000 

 feet. 



