A VISIT TO THE llAXaciIOW BORE 



1 1 1 



caused by the oscillation of the larger body of water into which the river 

 empties. The sea resembles a large pond in which the water rises and 

 falls with the oceanic tide, and a river is a canal leading into it. The 

 rhythmical rise and fall of the sea generates waves which would travel 

 up the river, whatever were the cause of the oscillation of the sea 

 and quite independent of any direct action of the sun and moon on 

 the water of the river itself. 



There are four characteristics of tidal currents in rivers which are 

 of cardinal importance in the present connection. Briefly treated, 

 thev are : 



1. Dependence of Speed on the Depth Alone. — It may readily be 

 shown mathematically that loner waves travel in shallow water at a 

 speed which depends only on the depth of the water, and that waves are 

 to be considered long when their length is at least twice the depth of 

 the water. Xow the tidal wave in a river is many hundreds of times 

 as long as the depth, and consequently it travels at a speed dependent 

 only on the depth of the river. Moreover, its speed is very slow com- 

 pared with the motion of the great tide wave in the open sea. 



2. Difference between Ebb and Flow in a River and alone/ an Open 

 ( 'oast. — On the oj3en seacoast ebb and flow are simultaneous with fall 

 and rise, but in a river the case is quite different. On an open coast 

 slack water occurs at high and low water, but in a uniform canal 

 connecting with the sea, slack water, I. c. the time of no tidal current, 

 is at mean water-level, the current being most rapid up-stream at the 

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



A Constricted Arching. Note the Lack of Prominent Keystone. 



