It is a curious reflection on the progress of science in our time, that a term which has only come into common use in aeronautics in the last decade commemorates the name of a man who carried out his researches some fifteen years before the Wright Brothers first flew in 1903. The obscurity surrounding Ernst Mach, whose name is given to the ratio of the speed of flow of a gas to the speed of sound under the same conditions, is ill-deserved, for there can be little doubt that he was one of the outstanding scientific personalities of the nineteeth century; this distinction arises not so much from his experimental work, which was extremely valuable, but more from his original conceptions on the philosophy of science which helped to lay the foundation for modern developments in physics.