314 WAVES OF THE SEA 



6 waves had a period of i second 

 6 2 seconds 



5 i> 3 



*4 ^ 4 11 



D 5 11 



5 M M O ,, 



4 11 > 7 11 



^ 1 " 11 



3 11 i 9 i 



i wave 10 



Reverting from figures to the description of 

 appearance, I may note the following additional 

 circumstances observed in the Griinnbach conduit, 

 which tend to elucidate the origin and mode of 

 growth of the train, or series, of roll-waves. 



The first noticeable agency of growth is the 

 catching up of the smaller waves by the larger, 

 due to more rapid motion of the latter ; but a 

 new factor (not possible on the cement floor of 

 the Glion conduit) is the increase of amplitude 

 as the front passes over each transverse ridge of 

 the flag pavement. It appears, therefore, that we 

 have here a case of a progressive wave increased, 

 not merely by friction with a flat bed, but also by 

 passing over perceptible transverse inequalities. 

 The depths being small, the reasoning previously 

 employed to explain discontinuous flow as result- 

 ing from local increase of depth still holds 

 good. 



