PREFACE. 



The Pacific coasts of Japan have from time to time been 

 invaded by destructive sea waves, the so-called ' tsunamis.' Our 

 history abounds with the records of terrible catastrophes 

 wrought by ' tsunamis,' which were frequently associated with 

 great eartliqnakes originating in the bed of the neighbouring 

 ocean. The damage to life was sometimes counted by many 

 thousands, villages were swept away in a moment, and some- 

 times large extents of coast lines were buried under the water. 

 Before inquiring into the means of mitigating the damages 

 associated with the horrible inflow of the waters of the ocean, 

 it has in the first place been necessary to discover tlic nature 

 of excitement of these waves, and afterwards to search after 

 the necessary appliances for alleviating the effects of these 

 catastrophes. It was with this object in view that the second- 

 ary oscillations of the oceanic tide had to be studied, for 

 solving the problem touching the mode of excitement of the 

 destructive sea waves. 



One of the most destructive sea waves of recent years was 

 that of Sanriku, by which the coasts of Eikuzen, Rikuchu, and 

 Mutsu bordering on the Pacific Ocean suffered serious damage. 



By examining the mareograms during the disturbances, 

 Prof. Omori was led to the conclusion that the bays or inlets 

 oscillate like fluid pendulums with periods peculiar to their own. 

 What is the mode of excitement and how the period could be 

 calculated remained still unknown. 



