Jets from notched nozzles are investigated by schlieren photography and pressure traverses. It is demonstrated that the dominant feature of the flow which determines the structure far downstream is the trailing vortices shed from the swept edges of the notches.

The density distribution in the relaxation regions of shock waves in carbon dioxide were determined in the Mach number range 1·4 to 4·0 using an interferometer. The over-all density ratios were found to agree with the theoretical final equilibrium values. Detailed analysis of the relaxation regions showed that the simple relaxation equation is inadequate, the relaxation frequency depending on departures from equilibrium as well as on temperature.

The physics of shock-waves with vibrational relaxation regions is recapitulated, and it is shown that exact methods of analysis can be developed from the classical Rayleigh-line equations by treating the real gas as an ideal gas with heat transfer. By using these methods to analyse experimental records of density distributions in relaxation regions, a large number of local values of the relaxation frequency, rather than a single over-all value, may be obtained from each shock-wave record.

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An approximate expression is given for the thickness of weak fully dispersed shock waves. Using available data on the thermodynamic properties of air, it is shown that shocks of the strength expected in sonic bangs are fully dispersed. Estimated relaxation times for dry and humid air lead to wide variations in possible thickness, varying from millimetres to metres.