222 WAVES OF THE SEA 



lated velocity of a " long " wave in different depths 

 of water be compared with the observed velocities 

 of current in large rivers, it will be seen at once 

 that the wave-velocities are many times greater 

 than the velocities of the currents. Thus the flood- 

 water obtains room for itself, not so much by rush- 

 ing forward as by causing the waters for a long 

 way in front to lift or swell slightly. The trans- 

 mission of pressure through the current is so rapid 

 that there is (except under special circumstances, 

 to be described later) no visible wave, but the river 

 lower down is found to be rising long before the 

 arrival of the floating and suspended matter which 

 accompanies the arrival of the actual waters of 

 the flooded tributary. The speed at which this 

 wave -transmission of the flood progresses down the 

 channel is equal to : 



Velocity of current plus velocity of " long " wave. 



In the case, however, of a shrunken river flowing 

 through alluvial soil the rate of advance of the 

 " first rise " of water after a drought may be much 

 diminished by the absorption of water by the 

 porous ground over which the river has to spread. 



When a river increases in depth in its 

 lower reaches, the wave -velocity increases, and 

 the amplitude of the wave is thereby dimin- 



