232 WAVES OF THE SEA 



tide considered as a single wave, although Sir G. H. 

 Darwin has pointed out that theory serves rather 

 to explain a rapid rise than an absolutely sudden 

 one. 



In the account which I have now to give of the 

 tidal bore, as I have seen it in the River Severn, 

 it will be found that it seems to have a wave -length 

 insignificant in comparison with that of the tidal 

 wave considered as a whole. 



Wave -Length of the Severn Bore 



The River Severn, which is tidal up to and 

 beyond Gloucester, pursues below this city a 

 sinuous course, similar to that of a non-tidal river 

 in an alluvial plain. The gradient is at first slight, 

 and the chief peculiarity of the bed is the occur- 

 rence at several places of ledges of rock lying 

 across the stream, which suddenly reduce its depth. 

 Thus the depth at ordinary low water over the 

 " Stonebench " is 3 feet, whereas the depth in the 

 long pool below is 10 feet. Denny Rock is another 

 shallow, which has a deep reach above it. The 

 character of the river changes somewhat near 

 Priding, where sand-banks begin to be prominent 

 at low water. Between Gloucester and Priding 

 is the best part of the river for viewing the bore, 



