iv PBEFACE 



Although a few historical notes are inserted here and there, 

 there is no attempt at systematic citation of authorities. The 

 reader who wishes to carry the matter further will naturally 

 turn in the first instance to Lord Rayleigh's treatise, where 

 full references, together with valuable critical discussions, will 

 be found. I may perhaps be allowed to refer also to the 

 article entitled " Schwingungen elastischer Systeme, insbeson- 

 dere Akustik," in the fourth volume of the Encyclopddie der 

 mathematischen Wissenschaften (Leipzig, 1906). 



I have regarded the detailed description of experimental 

 methods as lying outside my province. I trust, however, that 

 no one will approach the study of the subject as here treated 

 without some first-hand acquaintance with the leading pheno- 

 mena. Fortunately, a good deal can be accomplished in this 

 way with very simple and easily accessible appliances; and 

 there is, moreover, no want of excellent practical manuals. 



I have to thank Mr H. J. Priestley for kind assistance in 

 reading the proof-sheets. 



H. L. 



January, 1910. 



