Aero-mercurial Tide- Gauge. 259 



In the bay of Hososhima on the coast of Hiuga, extremely 

 regular undulations appear superposed on the tidal wave; the 

 period varies from 17*7 m to 20 m according to the tidal phase. 

 The period slightly decreases as the tide passes from the low 

 water to the high. In very calm weather, the amplitude of 

 undulation amounted even to 25 cm, This regular undulation 

 is a stationary oscillation having its node and loop at the 

 mouth and the end of the bay respectively, as actually shown 

 by Y. Yoshida, T. Terada and the author * in several bays on 

 the coast of Sanriku. The period of oscillation is therefore 

 given by the formula 



T- 4? 



where I is the length and h the mean depth of the bay. In the 

 present case, the calculated value is 19 m , which fairly agrees 

 with the observed one. The change of the period caused by 

 the tidal motion has also the range which is to be expected 

 from the above formula. 



In the bay of Aburatsu in Hiuga, we also observe con- 

 spicuous secondary undulations, though they are not regular. 

 The period varies from 15 m to 22 m ; the amplitude frequently 

 exceeds 13 cm. The calculated period is 17 m , which lies 

 within the range of the observed periods. 



The tidal wave at Kagoshima is generally very smooth ; 

 but frequently distinct secondary undulations of 19 m or 24 m 

 are observed. 



In the bay of Nagasaki, the secondary undulation is so con- 

 spicuous that it is usually known as "Abiki." The undulation is 

 not, however, regular; its amplitude often exceeds half a metre. 

 On one occasion, about ten years ago, the amplitude of the 

 abiki amounted even to two metres, and a large number of boats 

 and steamers are said to have been damaged. According to 

 my observations, the period irregularly varies from 25 m to 

 43 m . Judging from the form of the bay, several nodal lines 

 at the mouth are conceivable ; thus the largest and smallest 

 values of the calculated periods are 28 m and 41 m , whose 

 interval fairly coincides with the range of the observed 

 periods. 



March 30, 1905. 

 Tokyo, Japan. 



* Honda, Yoshida and Terada, loc. cit. 



S2 



