Although the subject of wave propagation in solids has a long history, the classical theory of elastic waves having been developed in the nine teenth century by STOKES, POISSON, RAYLEIGH and KELVIN, the last two decades have seen a remarkable revival of interest in this subject among both theoreticians and experimenters. There' are a number of reasons for this; first, experimental methods for the generation and detection of high frequency mechanical waves have become available only with the advent of electronic techniques and of high speed photo graphic recording apparatus. Secondly, the...