SECONDARY UNDULATIONS OF OCEANIC TIDES. 9 1 



and again increased ; at S*" 7"\ the largest wave invaded Miyako. 

 The subsequent large waves rolled on the shore at the following 

 epoch : — 



8" 15'", 8".32"\ 8" .48", 8\59"\ 9M6™, 9\50'"; 

 the intervals between two consecutive waves are 8™, 17"\ 16™, 

 11™, 17™ and 34.™ The period of the conspicuous undulation of the 

 bay of Miyako commonly observable in ordinary weather is 21™, 

 which is somewhat different from the period of the present sea 

 wave. From the above intervals, it may be concluded that the 

 sea wave w^as probably composed of the period of about IG™ 

 and its octave. /Without the aid of a tide-gauge, it would often 

 be difficult to observe every incident wave of 8™ out of waves 

 so composed. 



The record of the nearest tide-gauge was that of Ayukawa 

 (PI. LIV, Fig. 1) ; it shows a series of gigantic waves of the period 

 8™, which continued over two days ; during the first twelve hours, 

 the rising and falling of the level followed each other most ener- 

 getically. Comparing the period of the wave with that observed 

 at Miyako, it may be concluded that the period of the waves 

 incident on the bay nearly coincided with the period of its free 

 oscillation. Examining the record of the tide-gauge, we observe, 

 in the latter part of it, a beautiful series of beats which pro- 

 bably shows that the period of the incident wave is slightly 

 different from that of the free oscillation of the bay. 



The sea wave also shghtly affected the tidctgauge of Abura- 

 tsubo in Misaki (PI. LIV, Fig. 2), which is about 600 km. distant 

 from its origin. Its period of undulation was 15™ in a good 

 coincidence with that usually observable. The same w^ave also 

 affected the tide-gauge at Hakodate ; unfortunately, this instru- 

 ment stopped at the very beginning of the sea wave, and began 



