90 Messrs. K. Honda, T. Terada, and D. Isitani on the 
the great eruption of Krakatoa 1888, were also found to show 
the same periods as were frequently found in ordinary cases. 
He explained this interesting fact b}< supposing that a bay,, 
or a certain portion of the sea, makes some fluid pendulum 
motion with its own period. Professor H. Nagaoka*, in his 
paper on the hydrodynamical investigations of sea-waves, 
expressed the desirability of a special inquiry looking towards 
an explanation of this phenomenon. The suggestion was 
taken up by the Earthquake Investigation Committee, with 
the result that the task of making a series of systematic 
observations was imposed upon us. 
The observations were carried out during the years 3 903 
to 1906 ; the number of bays and coasts where work was 
done amounts to about sixty in all, extending from Hokkaido 
to Kiushiu. The work consisted firstly in finding the periods 
of the undulations peculiar to each bay or coast ; and 
secondly in comparing the phases of the different portions of 
a bay by simultaneous observations — which latter, however, 
was accomplished only for a number of the most typical 
bays. 
In most cases portable, self-recording tide-gauges (PI. I., 
figs. 1, 2) were used, the construction and theory of which 
has been already published in the Phil Mag. vol. x. p. 253 
(1905). 
§ 2. General Conclusions. 
The general conclusions f, which have been drawn from the 
thorough study of the numerous records obtained, are given 
in the following propositions. In discussing our results we 
have availed ourselves of the valuable records made by Lord 
Kelvin's tide-gauges at ten different stations. These instru- 
ments have been set up at various places on the coasts of the 
Pacific and of the Japan Sea, and some of them have been 
in working operation twenty years. 
1. On the Pacific coasts, free from any inlet in the coast- 
line, the secondary undulation is quite unnoticeable, 
and of a. very irregular nature. 
2. On the coasts of the Japan Sea the secondary undula- 
tion on the open coast is observable, though the periods 
of the undulations are not regular. 
* Nagaoka, Proc. Tokio Math-Phys. Soc. i. 1903. _ 
t In PI. II. a map of Japan is given. The locations of the principal 
observation stations are marked by points. 
