Secondary Undulations of Oceanic Tides. 103 
phase of the water particles in the bay was the same for all 
parts of the bay when the oscillation was fundamental. In 
an elongated bay, a binodal or trinodal oscillation was easily 
produced. 
It was convenient for the observation of the mode of oscil- 
lation to follow the motion of the fine cork powder, or better, 
fine aluminium powder which had been scattered over the 
surface of the water. To diminish the effect of surface 
tension of water as much as possible, a few drops of oil were 
poured on the water in the tank ; but before the fine powder 
was scattered over it, the water was well stirred. In this 
way the path along which the water particles moved could 
be easily traced. We also took the photograph of the model 
in the tank, when the bay water was oscillating. By placing 
a camera in a vertical position over the model and giving an 
exposure equal to about half the time of a period of oscillation, 
a photograph was made on which could be traced the path 
of each moving aluminium particle ; the aggregate of these 
paths then showed convincingly the actual mode of the 
horizontal motion. 
To determine the proper period of oscillation of a bay, 
the period of the pendulum was so adjusted as to give a 
nearly maximum amplitude of oscillation. The pendulum 
was then stopped, and the period of the subsequent oscil- 
lations was determined by means of a stop-watch. Though 
the period of the pendulum varied slightly from the above 
value, the period of the subsequent oscillation was quite 
constant. If the period of the exciter differed considerably 
from the proper period of the bay, the oscillation after the 
stopping of the pendulum was rapidly damped, and this 
gave us a good means of detecting whether the period of 
the exciter was near to the proper one, or not. 
We experimented with models of seven bays, in which 
regular and conspicuous undulations were observed : the 
results for three of these are given below : — 
(i.) Bay of Hakodate. 
The dimensions of the model were as follows : — Length 
1 : 20200, and depth 1 : 548, so that the factor r, by which 
the observed period in the model must be multiplied in order 
to obtain the period of oscillation of the actual bay, was 863, 
The bay had two modes of oscillation : the fundamental, with 
its node at the mouth of the bay, and the lateral, which 
oscillated between Hakodate and Tomikawa with its node 
midway between. The periods of these oscillations were 
3 a, 27 and l s, 64 respectively; multiplying by r we get 47 m, 
