268 WAVES OF THE SEA 



straight parallel ridges at right angles to the 

 current, quite across the stream, a form very 

 different from the wave -track of a ship. 



A solitary boulder in mid-stream does, however, 

 reproduce, very roughly, the wave-track of a ship, 

 a series of waves being formed on either side, 

 each wave inclined at an angle which appears to 

 be about the same as in the case of ship -waves, 

 viz., 19^ to the current, each succeeding wave 

 projecting somewhat farther into the stream than 

 its predecessor. There is generally but little 

 indication visible of any system of transverse waves 

 like the thwart-ship waves which are comprised 

 between the diverging, or " echelon," waves of a 

 ship's track. 



When river-waves arise from the retardation 

 caused by a rough bank, or where a stream 

 narrows, or where it enters upon a different 

 gradient, the wave -front is inclined to the bank. 

 If the stream be wide, the oblique standing waves 

 are of greater elevation near the bank, the height 

 diminishing farther out, the centre of the river 

 being smooth. If, however, the stream be narrow, 

 the standing waves originating from the opposite 

 banks are superposed in the centre of the stream, 

 and the combined crests there have a greater ampli- 

 tude than either separately. Thus the highest wave 



