THE principle of the method for the determination of the velocity of projectiles described by Taylor and Wark in the issue of NATURE for Dec. 27, p. 994, has been used previously, notably by Kampe de Feriet, who published a full description, illustrated with some excellent photographs, in 1925 (Mem, de l'Art. franc., vol. 4, p. 289). Using what was essentially a single but comparatively wide band of light, a continuous record was obtained of the flight of the projectile for a distance of 25 metres.