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 appearan