THE TIDES. 167 



opposite end at Land's End, and imagine some- 

 how a disturbing force causing 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 equi- 

 librium 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 upper- 

 most figure of this diagram (Fig. 24). 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 (represented 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 at Dover the current 

 begins to flow west through the whole of the 

 Channel. When it is mid-tide at Dover the tide 



