AND OTHER WATER WAVES 329 



which I had often found useful, of half-closing 

 the eyes, so as to render the outlines of the 

 stationary objects indistinct. I saw immediately 

 that roll -waves extending the whole breadth of the 

 conduit were dashing down the channel all the 

 time with great speed, and at intervals of not more 

 than half a second. Thus by varying the method 

 of looking, it became possible to appreciate clearly 

 the dual aspect of the phenomena of the flow. The 

 roll -waves were no insignificant part of the whole 

 flow, the apparent predominance of the stationary 

 waves being due more to optical advantage than 

 physical superiority. I made a similar observation 

 in the roughly-paved steep conduit which carries 

 the Verraye torrent at Veytaux (Switzerland) in 

 the lower part of the course below the lower road. 

 In deep streams the down-stream progressive 

 waves are difficult to observe, but I have no 

 doubt that they are normally present wherever 

 waves exist at all. If the eye be allowed 

 to follow the stream, it is often easy to see 

 some travelling inequalities of surface in the 

 deep water, especially when the light is low, but 

 it is not always clear whether they are truly waves ; 



rounding buildings from frequent, or even daily, visits will 

 probably notice many architectural features which escaped their 

 attention during the hours of busy traffic. 



