1 66 POPULAR LECTURES AND ADDRESSES. 



I ask you to think of the oscillations of water 

 in a trough. Look at this diagram (Fig. 24), which 

 will help you to understand how the tidal effect 

 is prodigiously magnified by a dynamical action 

 due to the inertia of the water. The tendency 

 of water in motion to keep its motion prevents 

 it from taking the figure of equilibrium. [A 



FIG. 24. Oscillations of Water in a Trough. 



chart showing the tides of the English Channel 

 was exhibited, from which it was seen that while 

 at Dover there were tides of 21 feet, there was 

 at Portland very little rise and fall.] Imagine a 

 canal instead of the English Channel, a canal 

 stopped at the Straits of Dover and at the 



