A quantity B giving the difference between the acceleration caused by the gravitational attraction of a spherical cap and the acceleration caused by the gravitational attraction of a Bouguer plate. i.e., of an infinite plate of the same thickness and composition as the cap. The spherical cap has a radius equal to the average radius of the Earth, a thickness equal to that of the Bouguer plate (usually taken as the elevation at the point of measurement) and an apex angle of 2^o 59'56" (the outer diameter of Hayford zone O). Bullard's term is subtracted from a Bouguer gravity anomaly to obtain the equivalent, modified Bouguer gravity anomaly.