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Page:Popular Science Monthly Volume 72.djvu/115

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111
A VISIT TO THE HANGCHOW BORE

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, they 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. Now 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 compared with the motion of the great tide wave in the open sea.

2. Difference between Ebb and Flow in a River and along an Open Coast.—On the open 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. e. 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

PSM V72 D115 Constricted arching without a prominent keystone.png

A Constricted Arching. Note the Lack of Prominent Keystone.