The speed of flight of an aircraft at high altitudes is still usually determined by the indirect method of measuring the dynamic pressure in flight. The acknowledged defects of this method—the effect of interference by near parts of the aircraft on the static pressure at the tapping point, the necessity of calibration of the pressure readings by tedious quadrilateral flights over measured ground distances or a suspended static head, the indeterminate effect of the Mach number at high speeds, inaccuracy of the ‘air temperature measurements owing to stagnation of the flow at the instrument, errors of the elastic pressure capsule owing to hysteresis and temperature variations, and the inaccuracy of the mean values obtained by averaging the measured dynamic pressures—have all contributed to raise a demand for a simpler and more accurate method of airspeed measurement.