NOT LONG AGO there was a protracted discussion of the velocity of sound, begun by Professor Morduchow's objection to the "lack of rigor" in the customary derivations and carried on by Professors Morkovin, Morduchow, and Millikan, and by Dr. Munk. None of these authors saw fit to define the velocity of sound before deriving its value; most of the discussion assumed the existence of a barotropic relation P — f(p)The present communication makes no original contribution; its only purpose is to call attention to the basic known results on the subject, which were not mentioned by any of the four disputants.