The stability of plane shock waves was measured in a shock tube by perturbing the primary shock wave, formed on rupturing the diaphragm, by means of thin wedges. A time-record of the shape of the shock wave after it passed the wedges and travelled along a channel of constant cross-section was obtained by Schlieren photography. Analysis of the photographs enabled the rate at which the shock wave recovered its plane shape to be determined and this, together with the detailed shape of the wave at various instants, was compared with the first-order theory of Freeman (although all the conditions assumed in the theory could not be faithfully reproduced in the experiments).For shock-wave Mach numbers of 1·165, 1·41 and 1·60, the time-rate of decay of the perturbations was found to agree quite well with the theoretical value, but the amplitudes of the perturbations were much larger than those given by the theory.The experiments failed to give reliable information about the decay of the perturbations after a large time, owing, it is believed, to flow separation from the sharp corners of the wedges which constituted an additional source of disturbance to the shock waves.

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The temperature of shock-heated carbon monoxide has been measured by an emission-absorption method using the infrared radiation of the fundamental band centred at 4·7 μm. Temperatures in the range 600 to 4000 K were measured. These limits were determined by the shock tube rather than the technique. Small discrepancies between the measured and theoretical temperatures are adequately accounted for by shock-tube effects.

At the very high flight speeds of ballistic missiles, re-entering satellites and meteorites, the air in contact with the body may attain very high temperatures and may even become ionised. For example, the air in the stagnation region of a low-drag ballistic missile re-entering the Earth's atmosphere can reach a temperature of 8000°K and the electron density may be as high as 1016 per c.c. In any experimental work on high enthalpy flow it therefore becomes necessary to have techniques for measuring these high temperatures and high electron densities. The aim of the present paper is to describe techniques for measuring these quantities.