WAVES OF THE SEA 231 



flood tide is, of course, zero, and ebb goes 

 on all the time. The phenomena are too extended 

 in a horizontal direction, and the surface gradients 

 of the water are, for the most part, too small for 

 the eye to see the ebb and flow of the tidal river as 

 a wave ; but in a diagram, with the vertical scale 

 much exaggerated, showing in profile the advance 

 of the tide up the river, it appears in the form of a 

 travelling wave, which becomes steeper as it 

 advances up the river. If the course of the ebb 

 tide down the river be similarly shown on a 

 diagram, it will be seen that the front of the wave 

 is never so steep as the front of the flood -tide wave 

 becomes in the upper reaches of the tidal river. 

 This is partly because at the commencement of 

 ebb the river is full and the water deep, instead 

 of being at its shallowest as at the commencement 

 of the flood. 



In many tidal rivers a visible wave with a very 

 steep front, travelling up-stream, constitutes the 

 " first rise " of the flood tide, which is sometimes 

 referred to by boatmen simply as the " head " of 

 the flood, but it has also specific names, such as 

 the " Bore " and the " Aegir " (on the Trent). 

 This phenomenon has long been accepted in works 

 which deal with the theory of tides as being the 

 steep face, or the steep part of the face, of the flood- 



