To include all the facts of which a knowledge is expected in elementary examinations. To present a clear picture of the external physical processes which cause the sensation of sound. As the mathematical symbols for a physical process are easily made a substitute for a clear conception of the process, the mathematical symbols have been avoided as far as possible. To keep before the reader the distinction between phrases which describe actual processes or conditions and phrases which, while they facilitate the prediction of real processes and real phenomena, do not themselves stand for any physical condition or event. Such phrases are used in every department of physics, and are only mathematical symbols in disguise. Where I have departed from, or added to, the usual forms of explanation, it has usually been with the intention of making this distinction clearer.