252 WAVES OF THE SEA 



sinuous, those of the flood broader, shallower, 

 and straighter. Tidal bores appear to be charac- 

 teristic of rivers where there are drying sand-banks, 

 and are perhaps only produced there in a part of 

 the sandy estuary which has not yet settled down 

 to a permanent form. The bore marks a struggle 

 between flood or ebb which appears to be a transi- 

 tory state of things, for in Professor Osborne 

 Reynolds's model estuaries the bore disappeared in 

 all cases when the sand-banks had attained their 

 final form. 



From the above it appears, when we study on the 

 spot the conditions of a tidal bore as exhibited 

 in the River Severn, that the phenomenon is some- 

 what more local and less general than the usual 

 mathematical treatment of the subject would lead 

 one to expect. The visible wave is not the 

 steepening of the front of the tide as a whole, but 

 the steepening of the front of the rising water in a 

 short reach of the river. The observation above 

 recorded of the multiplication of the wave on enter- 

 ing the water of moderate depth supports this view. 



The tide presumably does not make its way 

 steadily up a tortuous channel, its progress, on the 

 contrary, being alternately checked and hastened. 

 Even on an open coast the rise of the tide, as 

 registered on the gauges, is markedly pulsative, 



