SECONDARY UNDULATIONS OF OCEANIC TIDES. 101 



corresponding to the mean depth of the sea, is calculated to be 

 about 100 km., \vhJcli is not very small, if compared with the 

 dimensions of Japan Sea. The distance from Korea Strait to the 

 northern end of Hokkaido is only ten times this wave length. 

 Hence if a train of waves be generated in or propagated into 

 any part of this confined l)asin, it may be propagated impartially 

 along the entire coast without considerable dissipation and affect 

 all the tide-gauges on the coast. Such a train of waves may 

 also undergo repeated reflections and refractions, before it is 

 quite dissipated away, and cause the variety of records for 

 different stations. 



The similarity of the periods of waves accompanying 

 cyclonic storms may, in some measure, be observed on the 

 Pacific coast also. Besides, at different stations, though char- 

 acterized by the undulations peculiar to their own, the mare- 

 ograms present not unfrequently a common feature with respect 

 to the periods of exciting waves ; a fact which suggests the 

 similarity of waves generated by and propagated from cyclonic 

 regions. Another interesting fact is that the wave length of 

 the most prevalent cyclonic waves is generally comparable with 

 the dimensions of tlie area of the cyclonic depression. Though 

 we are not yet enabled to solve the problem on the exact basis 

 of hydrodynamics, it results from all probabilities that a baro- 

 metric fluctuation at a cyclonic center, acting in an impulsive 

 manner, may give rise to a train of long waves propagated 

 from thence, and that their wave lengths are comparable with 

 the variations at the center. N. Denison has sought to attribute 

 the secondary undulation to Helmholtz's " Luftwogen," but the 

 periodic variation of surface pressure is not the only means of 

 producing a train of waves. A single impulse often snflices to 



