and many others have been the subject of long and difficult scientific examination.
At the National Physical Laboratory there are five wind channels continually in use to test on models all the various factors on which the aerodynamic efficiency of a machine depends. Two of these channels are 7 feet in diameter and nearly 80 feet in length; in one, wind speeds up to 60 miles an hour can be obtained. The model is attached to a specially designed balance or dynamometer and the forces it experiences in various positions relative to the wind are measured; from these data the behaviour of the machine in flight is determined. Here Mr Bairstow and his colleagues have worked out the practical conditions of stability of motion and determined by many ingenious devices the constants which occur in the theory. That theory was first given in a general form by
