Secondary Undulations of Oceanic Tides, 101 
pressure may often occur, and that this barometric dis- 
turbance, giving rise as it does to waves of considerable 
length, may be a cause of the abiki. A deep barometric 
centre with regular concentric isobars, which is approaching 
the district, excites short waves of considerable amplitude, 
but does not cause an abiki of a marked amplitude. 
As to the modes of oscillation of the bay, two are con- 
ceivable. The one is the seiches between the Fukahori 
side and the end of the bay; the other the fundamental 
oscillation having its node at the mouth. The periods 
calculated on this supposition are 22 m -6 and 37 m *5 respectively, 
in good agreement with the observed periods : experiments 
with the model gave also fairly coincident values. 
Near the western mouth of the bay of Nagasaki lies 
Fukahori, where Kelvin's tide-gauge incessantly operates. 
The undulation is generally imperceptible ; the same periods 
as those at Nagasaki are also traceable. It is interesting to 
note that, though the uninodal oscillation of the bay is very 
prominent at Nagasaki, it is not especially so at Fukahori, 
the latter being situated near the node of the oscillation. 
Even the great abiki of Nagasaki on May 2, 1905, was only 
30 cm. in amplitude at Fukahori. On the other hand, the 
seiches between Nagasaki and Fukahori sides are conspicuous 
even at the latter station, where the oscillation nearly forms 
the loop. 
§ 4. Experiments with Models. 
To confirm our theory, we decided to experiment with 
models, in order to endeavour to find the actual mode of 
oscillation of the bay. Several models of bays were made 
exactly proportionate to the original, and the periods of the 
oscillating water in these models were compared with the 
observed ones. In reducing the period of oscillation in the 
model to the actual one, it was assumed that the period is 
proportional to its length, and inversely proportional to the 
square root of its depth, provided the latter is a small fraction 
of the former. 
The construction of a model proceeded thus : first, contour- 
lines of the bed of the bay to be modelled were drawn on 
separate zinc plates, and these plates cut along these lines. 
They were then placed one above the other, the space between 
each two being kept by blocks of wood of such thickness 
that the ratio to the actual depth was in each case maintained. 
The interspace between the plates was then filled with cement. 
The model thus constructed was immersed in a large rectan- 
