SECONDARY UNDULATIONS OF OCEANIC TIDES. 47 



bay. Since March 1905, a tide-gauge of our system has been 

 set up in the same place by the Office, and many beautiful 

 records obtained. 



In the bay, tlie secondary undulation is so conspicious 

 that it is usually known as ahiki. The observed periods are 

 22.5™- 25.2", 31.9"- 32.4", 34.5"-37.6"\ 40.1", 44.5"- 45.2", 53.G" 

 and 69.0"- 72.0"; the amplitude of the conspicuous undula- 

 tions often exceeds half a meter. On one occasion, about 10 

 years ago, the amplitude of the ahild was over 2 meters, and 

 a large number of boats and steamers are said to have been 

 damaged. The largest amplitude since the beginning of the 

 tide-gauge observation was 1.54 m., at midnight on May 1, 

 1905. PL XLIV and XL Y are records of the famous abild 

 together with records of less remarkable undulations. 



The conspicuous ahiki is generally associated with weather 

 in which the isobars in the neighbourhood of the district is 

 much crooked by two coexisting low barometric centres (PI. 

 XCIII — XCIV). It is well known that a tornado is frequently 

 associated with such a distribution of isobars ; then it seems 

 very probable that a sudden local disturbance of pressure may 

 often be accompanied with an unstable baromomotric distribu- 

 tion. The ahild is often found in apparently calm weather ; a 

 deep barometric centre with regular concentric isobars, which 

 is approaching the district, excites short waves of considerable 

 amplitude, but does not cause the ahiki in a marked degree. 



As to mode of oscillation of the bay, two are conceivable. 

 The one is seiches between Fukahori and the end of the bay ; 

 the other is the fundamental oscillation having the node at the 

 mouth. The periods calculated on this supposition are 22.6" and 

 37.5" respectively, closely agreeing witli the observed periods ; 



