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 Encyclopadz'e der mathematischen Wissenschaften (leipzig.