318 WAVES OF THE SEA 



me that no obvious change in the waves occurred 

 when the gradient diminished. 



The uniformity of depth in this conduit is, of 

 course, a most important factor in preserving the 

 transverse waves, and the unequal depth (in cross- 

 section) in natural streams equally tends to prevent 

 their development. I saw an excellent example 

 of this at St. Maurice, in the Rhone Valley, where 

 a paved conduit terminated in a channel encum- 

 bered by gravel, through which the stream had 

 made for itself the usual winding course, with 

 unequal depths on the two sides. The conduit 

 delivered strong transverse roll -waves at intervals 

 of a few seconds, each of which vanished almost 

 instantaneously upon entering the winding channel 

 of unequal depths in cross -section. 



Since the velocity of the wave relatively to the 

 banks is that of the current plus the true wave- 

 velocity, and the latter is greater for deep water, 

 it follows that the wave -front loses its trans - 

 versality when the cross -section of the stream is 

 not uniform, and this is more marked in down- 

 stream roll -waves than in up-stream bores (where 

 current and wave oppose one another). This is 

 one reason why down -stream progressive waves are 

 so much less familiar than up-stream bores. 



