266 WAVES OF THE SEA 



equal to that of the stream. 1 This conception will 

 be found very helpful to a proper understanding 

 of the subject ; and that it is no mere subtlety of 

 thought is realised immediately by one who shall 

 drift in a boat through such a train of waves. 

 Dropping down -stream in this way on the ebbing 

 tide between the piers of the Severn Bridge opposite 

 Sharpness, my eyes were fixed upon the waves 

 which combed over towards me. Almost at once 

 the sense of drifting vanished, the boat seemed no 

 longer to progress through rough water, but, on 

 the contrary, to be at a standstill, whilst wave 

 after wave charged past her. 



In small streams it is not difficult to produce 

 stationary waves by introducing a stone or 

 boulder, or, better still, a transverse barrier which 

 rises above the bottom sufficiently near to the 



1 An excellent illustration of a wave with a motion equal and 

 opposite to that of the current is seen when one pours water 

 into the centre of a flat-bottomed circular sitz-bath. The 

 current flows outwards in all directions with diminishing 

 velocity, and the water, resurging from the circumference, makes 

 a breaking wave with a circular front facing the centre of the 

 bath. As the depth of water behind the wave increases the 

 velocity of the wave also increases, so that its front closes in 

 towards the centre where the outflowing current is more rapid. 

 The circle is presently reduced, at first slowly, afterwards 

 more rapidly, so that only a small hole is left, and finally the 

 wave closes in completely upon the down-pouring column of 

 water. 



