CHAPTER VIII 



Tidal bores Wave-length of the Severn bore On the want of 

 concordance between height of tide and height of bore, 

 and on the conditions which determine the starting-point 

 of the bore. 



On Tidal Bores 



THE time during which the tide flows in a river 

 is less than that during which it ebbs, for during the 

 flood tide the river quickly gathers an opposing 

 " head " of water (both on account of the slope 

 of its bed and of the diminution of its breadth) 

 which reverses the tidal flow, so to speak, before 

 its time. Conversely, the form of the channel 

 as .well as the momentum of the land water, or 

 true river water, prolong the seaward flow of the 

 ebbing tide. If the conditions be considered at 

 different places successively farther up the river, 

 it will be seen that this difference increases, 

 the duration of flood tide being less and that of 

 ebb tide greater at places higher up than at 

 places lower down the river. When we reach the 

 limit of tidal influence of the river the duration of 



