When an observer moves over the face of the earth, there is a change in both the magnitude and the direction of the apparent acceleration due to gravity. At speeds now becoming attainable, the change is just about observable with modern aircraft navigating instruments. The change consists of two terms—a Coriolis term and a centripetal term. If a particle could be dropped through a vacuum from an aircraft flying a great circle course, the Coriolis deflection of the falling particle,as seen by the airman, would be independent of the aircraft's velocity.