SECONDARY UNDULATIONS OF OCEANIC TIDES. 51 



the amplitude of oscillation of the pendulum gradually decreased, 

 if it were once started and left to itself, it could be kept fairly 

 constant by applying a small force by hand at suitable intervals. 



To avoid the reflection of the excited wave from the walls 

 of the tank, a tliick layer of a damping material, such as 

 wood shavings, was laid in front of the reflecting walls. 



By exciting waves with tlie above arrangement, the water 

 in the model of the bay made a standing oscillation, whose 

 amplitude was generally small ; but as the period of the pendulum 

 approached the proper period of the bay, the amplitude of oscil- 

 lation gradually increased, and when the period exactly coincid- 

 ed with that of the model the amplitude of the latter was 

 a maximum. In this case, the mode of oscillation proved to 

 be that conceived by us, that is, the end of the bay was a 

 loop for vertical motion and a node for horizontal motion, 

 while its mouth was a node for vertical motion and a loojj 

 for horizontal motion. The phase of the water particles in the 

 bay was the same for all parts of the bay, when the oscilla- 

 tion was the fundamental one. In an elongated bay, a binodal 

 or trinodal oscillation was easily produced. 



For observing the mode of oscillation of the water, it was 

 convenient to follow the motion of line cork powders or better 

 fine aluminium powders scattered over the surface of the water. 

 To diminish the effect of surface tension of water on the motion 

 of the powders as much as possible, drops of oil were put 

 into the tank ; the powder was then finely scattered, the 

 surface of water being well stirred. 



In this way, experiment with the model showed clearly the 

 paths, along which the water particles moved. We also took 

 photographs of the model in the tank, when the bay water was 



