AND OTHER WATER WAVES 227 



The arrangement of a river in alternating pools 

 and shallows is a condition theoretically favourable 

 to the formation of such roll-waves as are de- 

 scribed by these observers. The wave -velocity 

 diminishing greatly where the bottom rises at the 

 lower end of the pool, this section at first receives 

 far more than it can pass on, so that the wave 

 face, instead of being of infinitesimal gradient, 

 becomes quite steep a " wall " of water in 

 appearance. 



On entering a pool the wave may perhaps lose 

 this form, commonly called a bore, and become 

 a group of rounded swells, but on entering 

 shallow water this will close up, and the front of the 

 disturbance will be either a cusped wave, like a 

 wave about to break upon the shore, or a solid 

 wedge of foaming water the bore proper like 

 those beautiful foaming ridges which come in upon 

 a flat sandy shore when the great waves of a storm 

 are breaking far out at sea. 



Roll -waves are also reported as occurring in the 

 Rhondda and the TafT Rivers in Wales. They 

 are frequent in the rivers among the foothills of 

 the Himalayas. They are also recorded from time 

 to time as the result of an exceptional and solitary 

 cause of flood in various parts of the world. Thus, 

 during the great typhoon in the autumn of 1902 



