180 WAVES OF THE SEA 



On a steeply sloping beach obliquely breaking 

 waves are even more common and conspicuous. 

 It is scarcely necessary to insist upon the fact that 

 longshore drift due to wind will depend upon the 

 length of fetch of the wind as well as upon its 

 velocity, since large waves are only produced by 

 the action of wind upon a long stretch of water. 



The effect of the tides to determine the 'long- 

 shore drift of beach shingle by waves is, on the 

 other hand, a complex matter, and it is of the 

 highest importance that the general principles 

 governing this effect should be definitely laid 

 down. I will endeavour to do this for our 

 own coasts. 



Twice a day a current flows from the Atlantic 

 Ocean into our narrow seas, and produces a rise of 

 water-level there. Wherever the tide runs freely 

 the currents follow the same rule as the forward 

 and backward currents of the ordinary wind -waves 

 of the sea, viz., when above the mean sea-level 

 they are forward ; when below mean sea -level, 

 they are backward. 



Thus, wherever the tide runs freely along a coast 

 e.g., on a straight shore or past a headland 

 the current is what we may call up -channel during 

 the whole time that the water is above mean sea- 

 level. This means eastwards for the English 



