1. For a recent review of this subject, see M. J. Lighthill,Surveys in Mechanics, G. I. Taylor 70th Anniversary Volume(Cambridge University Press, New York, 1956), p. 250.
2. The notation used here conforms with W. Hayes inFundamentals of Gas Dynamics, edited by H. Emmons (Princeton University Press, Princeton, New Jersey, 1958) Vol. 3, p. 451.
3. In fact for problems with viscosity negligible Eq. (6) also simplifies the classical theory of finite-amplitude waves, as shown by a comparison with H. Lamb,Hydrodynamics(Dover Publications, Inc., New York, 1945), p. 481.