THE TIDES 301 



Look at this diagram (Fie. 128), which will help you to 

 tuiderstand 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 chart showing 



FIG. 128 Oscillations of Water in a Trough 



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 opposite end at 

 Land's End, and imagine somehow a disturbing force caus- 

 ing the water to be heaped up at one end. There would be 

 a swing of water from one end to the other, and if the 

 period of the disturbing force approximately agreed with 

 the period of free oscillation, the effect would be that the 

 rise and fall would go vastly above and below the range 

 due to equilibrium action. Hence it is we have the 21 

 feet rise and fall at Dover. The very little rise and 

 fall at Portland is also illustrated in the uppermost figure 

 of this diagram (Fie. 128). Thus high water at Dover is 

 low water at Land's End, and the water seesaws as it were 

 about a line going across from Portland to Havre (repre- 

 sented by N in the figure) ; not a line going directly across, 

 however, for on the other side of the Channel there is a 

 curious complication. 



At the time of high water at Dover there is hardly any 

 current in the Channel. As soon as the water begins to fall 



