PREFACE 



In 1895 I went to live on the South Coast, and 

 every day the waves of the sea — beautiful, 

 mysterious, and insistent — drew me more and more 

 to the path on the cliff whence I could watch them 

 curl and break, and listen to their splash upon the 

 sandy shore. I stood there on the afternoon of 

 a calm day in early autumn at the time of low 

 water of a spring tide. The little waves, gliding 

 slowly in over the flat sands, bent round the ends 

 of a shoal, as waves of light are refracted, and, 

 meeting, passed through each other, each to con- 

 tinue its own course. Elsewhere a long, low wave, 

 impinging obliquely on a small bank of sand, was 

 thrown backwards at an angle of reflection equal 

 to that at which it had struck the obstacle. And 

 as I watched I thought what a fine thing it would 

 be if the study of all kinds of waves could be co- 

 ordinated. I had just finished another piece of 

 work, so I embarked at once on a course of reading 

 on waves of all kinds, which was, moreover, not 

 altogether new ground to me. I found that the 

 ideas which Newton and his successors have so 



