SECONDARY UNDULATIONS OF OCEANIC TIDES. 49 



undulation is not very conspicuous ; the observed periods are 

 25.3"-29.6" and 57.2™. The calculated period 25.8" fairly coin- 

 cides with tlie observed. 



(3) Takow. PI. XL VIII, Fig. 3-4. 



Takow is situated on the southern coast of Formosa ; here 

 Kelvin's tide-gauge is also constantly working. The undulation 

 is very inconspicuous; the periods observed are 11.9"- 13.7'" 

 and 24.4"-26.5'". 



§ 5. EXPERIMENTS WITH MODELS. 



To confirm our theory, it appeared interesting to experiment 

 with models, and thereby find the actual modes of oscillation of 

 different bays. Models of several bays were formed with definite 

 proportions to the actual dimensions, and the periods of the 

 models were compared wath those observed in bays. In reducing 

 the period of the model to the actual one, it was assumed that 

 the period is proportional to length and inversely proportional 

 to the square root of depth, provided the latter is a small frac- 

 tion of the former. 



To construct a model, contour lines of a bay were drawn 

 on separate zinc plates, wdiicli were afterwards cut along these 

 lines. The plates were then placed one upon another ; the 

 distance between two consecutive plates was kept by blocks 

 of w^ood of such thickness that it bears a definite ratio to the 

 actual depth. The interspaces between the plates were then 

 filled with cement. The models thus constructed were immersed 

 in a large rectangular tank (150 x 7Gxl9 cm.^) filled with water 

 up to the water line of the model. 



The waves were excited by a pendulum bob oscillating in 



