AND OTHER WATER WAVES 267 



surface. A mound of water forms in the neigh- 

 bourhood of the obstruction (its precise position 

 relatively thereto depending upon the circumstances 

 of depth and speed), and a hollow and a second 

 crest, and then a third, a fourth, and so on, appear 

 farther down-stream, a train of waves speedily 

 being formed. If the obstruction be now dragged 

 up-stream, the whole train of waves moves up- 

 stream with it, preserving their position relatively 

 to one another (which is the second connotion of 

 the epithet " stationary " as applied to them). In 

 performing this simple experiment the close re- 

 semblance between the ordinary waves of rivers 

 and ship-waves becomes at once apparent. The 

 analogy is yet further brought home when one 

 notices the waves formed by a model or toy vessel 

 moored in a rapid stream. The familiar train of 

 waves shown by a ship steaming through still water 

 is here reproduced in the water which flows past 

 the stationary vessel. 



There are, however, considerable differences in 

 the appearances usually presented by ship-waves 

 and river -waves, which are due partly to the fact 

 that the latter are produced by disturbances which 

 affect the water throughout its whole depth, and 

 the former are generally only superficial. Again, 

 river -waves, when caused by a weir, extend in 



