PREFACE. V 



communicating through a narrow channel. This is beautifully 

 illustrated in the oscillations of some bays, fulfilling the stated 

 condition. 



When the volume of water in the bay and consequently 

 the depth and the breadth change, it is necessary to add a small 

 correction. This question was discussed by Dr. Isitani. 



In limnological work, Sarasin's limnimeter is universally 

 recognised as a trustworthy and convenient instrument, and was 

 used in some of our lake surveys. In observations of secondary 

 oscillations, the large range of the tidal fluctuation prevented 

 the use of the instrument. The tide gauge in its usual form is 

 too cumbrous as a portable instrument, and the great damping 

 tlu'ough communicating tubes annihilates waves of secondary 

 oscillations of short periods, so that they are mostly lost on the 

 record. This defect was first modified by Dr. Nakamura by 

 balancing the pressure by means of a mercury column, and 

 greatly reducing the range of the tidal fluctuation without in the 

 least interfering with damping, which it was necessary to keep 

 suitably small in order that the secondary oscillations may not 

 be lost to view. This w^as further improved by Dr. Honda, 

 who greatly simplified the apparatus, and changed it to a neat 

 portable form. In most of the present investigations, Dr. Honda's 

 instrument was used. 



The records obtained either by the tide gauge or with Dr. 

 Honda's instrument all present semidiurnal fluctuations, and on 

 it the undulations to be investigated appear in serrated form. 

 For the exact study of the phenomena, it is sometimes 

 necessary to eliminate the tidal undulation. This was effected 

 by means of Dr. Terada's tidal rectifier, by which only the 

 secondary oscillations were brought to view. When the bay 



