154 OUR HERITAGE THE SEA 



the period of rest when the tide has reached its 

 highest or its lowest point, and which is usually only 

 a few minutes in duration, is prolonged to half the 

 time of a normal ebb and flow, three hours. Of 

 course, this prolongation of slack water, to the great 

 benefit of all concerned, varies in different places, but 

 it has a very marked effect upon the prosperity of 

 any port thus favoured, if only those interested are 

 sufficiently wide awake to take advantage of it. 



And now we come to one of the most extraordinary 

 cases the world can afford of the way in which one 

 tidal wave can neutralize another, one that causes the 

 observer to stand and wonder at the amazing develop- 

 ments of the forces of Nature. It has before been 

 noted how great openings, like that of the Bay of 

 Fundy, for instance, lying invitingly in the path of 

 the incoming tidal wave, do lend themselves to ab- 

 normal risings of tide, and how perfectly natural it 

 is that they should do so. Now in South- Western 

 America, wide open to the south-east, lies the great 

 estuary of the La Plata river. It is 150 miles wide, 

 and compared with it the opening of the Bay of 

 Fundy is but a creek. They both face the same way 

 nothing obstructs the full ingress of the Atlantic 

 tidal wave. Yet whereas, as we have seen in the Bay 

 of Fundy, the tide rise to the amazing height of 

 60 feet, in the estuary of the La Plata there is 

 practically no tide at all. The effect that this 

 marvellous tidal abnormality has upon the trade of 

 the country is made evident by a glance at the 

 statistics of shipping at Buenos Ay res, but the causes 

 of it are less easy to seek. According to the most 

 competent observers, this calling of a halt, as it were, 



