Secondary Undulations of Oceanic Tides. 93 
In the oscillation of the bay-water just referred to the 
period of the forcing wave, which corresponds to the maximum 
resonance, is not sharply defined, but within a small range 
the oscillation remains fairly conspicuous, as we have often 
proved. 
In the bays of regular shape, such as Of unato and Hoso- 
shima, the position of the mouth line is determinate ; but in 
the bays of complex shape, such as Shimoda and Susaki, 
several mouth lines are conceivable. By the choice of the 
mouth lines, the length and mean depth of the bay vary 
within a considerable range, so that the period of the proper 
oscillation changes within a certain range. Hence such a 
bay may resonate to any one of the incident waves whose 
period falls within the same range. In the two bays above 
mentioned, the period of the conspicuous undulation was 
actually found to vary within a wide range. 
As to the cause of the long waves, which manifest them- 
selves as secondary undulations, we may mention the wind, 
the cyclone, the earthquake, &c. It is a matter of fact that 
the seiche in many lakes, which is the result of interference 
of direct and reflected waves of long wave-length, is often 
excited by a strong wind. In the same way the wind blowings 
on the surface of the ocean may cause long waves of several 
kilometres. Such waves, too, are often caused by a deep 
cyclonic centre. Near such a centre fluctuations of pressure 
and of wind velocity go on incessantly, and these varying in- 
fluences, acting in an impulsive way, may cause waves of long- 
periods. An upheaval or depression of the sea-bottom, due 
to an earthquake or to a submarine eruption, may also be a 
cause producing the same result. 
§ 3. Results in Detail. 
In the present abstract, it is not proposed to give the 
details of observations made at all the stations ; we will 
confine ourselves to mentioning some of the remarkable 
findings in the bays, in which the secondary undulations 
appeared most markedly, and out of the sixty stations at which 
operations were carried on, we have chosen the following : — 
(i.) Hakodate, 
Hakodate, situated on the middle coast of the strait of 
Tsugaru, which separates Hokkaido from Honshiu, is the 
best anchorage in Hokkaido. The bay is approximately 
semi-circular in shape. Here Kelvin's tide-gauge has been 
continuously working during the last twenty years, and has 
