On the Motion of a Gas in Two Dimension

A system of lenses etc., is combined in such a way, that the portions in the jet where there is expansion appear white, and such parts where there is compression appear dark. The mouthpiece is constructed so that the stream-lines are parallel when leaving the opening. The experiment is arranged so t h a t the velocity of t h e gas in some parts of the mouthpiece is greater than the velocity of sound.8 Within the mouthpiece the pressure of the gas is greater than the pressure of the outer air.