Secondary Undulations of Oceanic Tides. 121 
been noticed by several earlier writers, such as Milne, 
W. J. H. Wharton, E. Geinitz, C. Davison, &c. The point * 
noticed by C. Davison as regards the mean velocity, if it be 
taken into account, does not diminish the discrepancy more 
than one per cent. 
(iii.) Sea-waves accompanying Cyclonic Storms. 
Disturbances of the sea-level caused by cyclonic storms 
produce phenomena which may be conveniently classified 
under three heads, namely, short waves ; long waves ; and the 
abnormal rise of sea-level. 
The violent, short waves, commonly called in Japan Gekiro, 
have periods usually of a few minutes, and are superposed by 
waves of still shorter periods but of considerable amplitude. 
These waves are always associated with strong gales, and have 
probably the same origin as ordinary win d- waves f. 
As to the long waves, examples are very numerous in 
which secondary undulations of remarkable amplitude occur, 
accompanying the cyclonic centre which was passing the 
vicinity J. In some cases remarkable undulations occur for 
a few hours, but soon abate into usual waves with the rapidly 
retreating cyclonic centres ; while in other cases they last for 
one or two days with considerable amplitude. In any case, 
the periods of remarkable waves in a bay are generally the 
same as those observed on ordinary days. The period of the 
most prominent undulation, however, is different in different 
cases : in some the fundamental period is most pronounced, 
while in others one of the higher harmonics is conspicuous. 
Generally speaking, the duration, as also the period of the 
prominent undulation, seems to depend on the width, the 
velocity, and the distance of the cyclonic centre. Again, in 
many cases, especially for the Japan Sea coast, the periods of 
conspicuous waves accompanying cyclones in different stations 
and for different occasions are nearly similar. On the Japan Sea 
coast it is frequently found that a series of waves recorded at 
one station is quite similar in form to one obtained at another, 
distant station on a different occasion. Similar cases also 
appear with regard to the Pacific coast, though often masked 
by the undulation proper to the bays. In connexion with 
this fact, it will be noticed with interest that the wave-lengths 
of the most prevalent, cyclonic waves are generally com- 
parable with the dimensions of the area of the cyclonic 
* Davison, Phil. Mag. vol. 1. ^1900). 
f Wheeler, ' Tides and Waves,' Ch. X. 
X See also ibid. Ch. XI. 
