An instrument is described for determining both the direction and the velocity of a steadily and slowly moving fluid. Use is made of the cooling effect on the sides of a symmetrical heated body exposed to the fluid stream. For measuring angles of yaw, the outputs of thermopiles at each side of the body are connected in opposition and a position of the instrument found which gives zero net output; for determining velocities the individual outputs are added. This simple and robust instrument is suitable for use in water at velocities in the range 30-300 mm s⁻¹ and in air at velocities of 0·4-6 m s⁻¹.