SECONDARY UNDULATIONS OF OCEANIC TIDES. 17 



excited by wind. In the same way, the wind blowing on the 

 surface of the ocean, may cause waves several kilometers long. 

 Long waves of a considerable amplitude are often caused by a 

 deep cyclonic centre. Near a cyclonic center, the fluctuation of 

 pressure or of wind velocity are incessantly going on, and this 

 acting in an impulsive way, may cause waves of long periods. 

 An upheaval or depression of sea bottom due to an earthquake 

 or to a submarine eruption, may also be a cause of sea waves 

 of considerable periods. 



§ 4. SPECIAL RESULTS. 



In the present section, the results regarding each special 

 bay will be described. Here it w^ill be remarked that the num- 

 ber of observed periods are naturally rich at stations in wdiich 

 the observation was continued for a long interval, while they are 

 poor at stations temporarily observed during one or two days. 

 The general results, wdiicli we have summarized in the foregoing 

 section, were deduced from the following results of observations. 



I. COASTS OF HOKKAIDO. 



(1) Otaru (Aug. 12-15, 1005). Top. 1. PI. 1, Fig. 1. 



Otaru is a city situated on the northern coast of Hokkaido. 

 The observation was made on the middle shore of the harbour. 

 Here, as in other coasts of Japan Sea, the tidal range is very 

 small, not exceeding 30 or 40 cm. ; the secondary undulation is 

 conspicuous, but not very regular. On the western side of the 

 harbour, Kelvin's tide-gauge is constantly working ; comparing 

 the record of the instrument with that of ours, we may con- 

 clude that the principal motion of the water in the harbour is 

 in the same phase. 



