10 PREFACE 



marvellously developed in the theory of waves 

 originated in the common knowledge of the "un- 

 dulating inequalities," as Dr. Johnson calls them, 

 which wind makes upon the sea. Periodicity, and 

 the transmission of an impulse by the material, are 

 the aspects of sea waves which are repeated in 

 the transit of light and sound, and owing to these 

 analogies we speak of " waves " of light and sound, 

 although a corrugated surface of progressive in- 

 equalities is no part of their character. 



I found, however, that while the physicists had 

 applied with great success the ideas got from the 

 aspect of the heaving sea to the study of sound, 

 light, electricity, and other matters, geographers and 

 geologists had not sufficiently availed themselves 

 of these ideas in the study of certain natural in- 

 equalities which, from their form and movement, 

 are as nearly related to sea waves in one way as 

 the pulsations of sound are in another. I also 

 found that the mathematical study of the waves of 

 the sea had been pushed nearly as far as the avail- 

 able data allowed, so that further progress in their 

 study could best be secured by observations accom- 

 panied by measurement, which I was capable of 

 undertaking. 



Thus having surveyed the whole subject of 

 waves, I decided to confine myself for the pur- 



Syr 



