In the October edition of the Journal Bragg and Smith contribute a stimulating article suggesting that in several examples of confined or two-dimensional flow pressure waves have a special velocity of propagation that is not equal to , the wave velocity in an infinite unbounded medium. Moreover phenomena such as choking can be said to occur at the special wave speed appropriate to the particular circumstances, so that it might appear sensible when describing the flow to use a Mach number based on the special wave propagation velocity rather than on They invite comments on this suggestion.