A VISIT TO THE HANGCHOW BORE 113 



3. Effect of Proper Current of the River Itself. — A river has also 

 to deliver a large amount of water into the sea during a single oscilla- 

 tion of the tide, and its own proper current is superposed on the tidal 

 currents. Hence in actual rivers, while the resultant current con- 

 tinues to flow up-stream after high water is reached, with falling 

 water-level, it ceases flowing before mean water-level is reached; and 

 while the resultant current ebbs down-stream after low water, it 

 generally continues to ebb with the rising tide for some time after 

 mean water is reached, the downward stream, in fact, lasting longer 

 than the upward one. The moments at which currents change will 

 differ in each river according to the depth, the time and the extent 

 of the rise and fall at the mouth and the volume of water delivered by 

 the river; but in every case the tide rises more quickly than it falls, 

 so that the time-interval from low water to high water is shorter than 

 from high to low. 



4. Natural Change in Shape of a Wave Advancing into Shallow 

 Water. — The demonstration is too technical to be included here, but 

 it can be proved analytically that a wave progressing up a river must 

 change its shape so that the front slope gets increasingly steeper, and 

 the rear slope more gradual. If this steepening of the front slope 

 be carried to an extreme, the wave would present the form of a wall 

 of water, but the mere advance into shallow water would not by itself 

 suffice to produce so great a change of form without the aid of the 

 natural current of the river, which cooperates with this change in the 

 shape of a wave as it runs into shallow water, so as to exaggerate the 

 steepness of the front slope. When, as is the case for many rivers, 

 the estuary contains broad flats or shoals of mud or sand which are 

 nearly dry at low water, the tide sometimes rises so rapidly, especially 

 if the mouth of the estuary be funnel-shaped, that the wave becomes 

 a wall of water, and is then properly called a " bore." Let us note 

 briefly the way in which Hangchow Bay affords typical circumstances 

 of this sort, so that we there have a most striking case of this interesting 

 phenomenon. 



The Physiography of Hangchow Bay 



Hangchow Bay, or the estuary of the Ch'ien-tang Biver, has a very 

 marked funnel shape. From Yangtse Cape (the extremity of Pu 

 Tung Peninsula) on the north to Ketau Point on the south is con- 

 siderably over sixty miles, while the distance between banks at a point 

 thirty miles farther west is approximately only half of this and in 

 twenty miles more has again been reduced by half, so that along the 

 meridian of Chapu it is only about eighteen miles wide. From Ketau 

 Point in a line approximately northeast there extends for over eighty 

 miles a chain of rugged islands, beginning on the south with Chusan, 



vol. lxxii— 8 



