Secondary Unci illations of Oceanic Tides. 105 
(iii.) Bay of San Francisco. 
During the last fifty years the tide-gauge at San Francisco 
has recorded several sea-waves originated at different coasts 
of the Pacific, the periods of which are 17 m *3-19 m *2, 24 m 3- 
27 m, 8, 34 m -3-41 m '2, 47 m '4 and 116 m , of which the first is an 
octave of the third. 
Now the bay is so irregular in shape that it is very difficult to 
find out by calculation what modes of oscillation correspond 
to the actual periods ; hence a model * was constructed and 
experiments made. The scale of proportion used was as 
follows :— Length 1 : 40000, and depth 1 : 366, so that the 
factor r was 2076. The model was too large to go in the 
tank, so it was placed in a small pool in the court of the 
University. 
Since the greater part of the model was very shallow, the 
oscillation rapidly subsided when the exciting wave was 
stopped, so that the period was always determined by ob- 
serving the maximum resonance of the bay. For the wave 
incident upon Golden Gate, the principal modes of oscillation 
of the water were those between West Berkeley and Sausalito 
sides. The remaining portion of the bay, including both 
ends, seems to have but little influence on these modes of 
oscillation. 
By exciting waves of the periods ranging from 3 s ' 1 to 
3 s * 5, the w r ater in the bay oscillated with the fundamental 
mode of oscillation, having its node near Golden Gate and 
its loop at West Berkeley side. The mode of oscillation most 
easily excited was a binodal seiche between the narrowest 
mouth-line and West Berkeley side. The period of the wave, 
which gave a maximum resonance to the binodal seiche of 
the bay, ranged from I s * 1 to l s '4. By slightly changing 
the period of the wave, the corresponding displacement of 
nodal line was observed. We could also produce a trinodal 
seiche of the bay, the period of which was nearly S, 8. 
Multiplying these periods by r, we get 107 m -122 m , 38 m -18 m , 
and 28 m . The period 116 m , which in all probability corre- 
sponds to the fundamental oscillation of the bay, was actually 
observed in the bay in the case of the sea-wave of South 
America, 1868. The periods corresponding to the binodal 
and trinodal oscillations above described have often been 
observed in the bay. 
In concluding the present section, it may not be out of 
* We used the chart published by the Washington Coast and Geodetic 
Survey and presented to Professor Umori by Dr. O. H. Tittmann, Super- 
intendent of the Office. 
