An approximation of the Bullard B--earth's curvature--gravity correction is expressed in terms of fractions of the standard Bouguer slab terrain correction. The Bullard B correction is appropriate for either high-resolution surveys or for rugged terrain such as the Rocky Mountains, where the correction can be as much as 1.5 mGal. The approximation has three terms, each separated from the others by roughly an order of magnitude, indicating a substantial segregation of physical effects. An analysis indicates that physically the three correction terms are for the curvature effect, the Bouguer slab truncation, and the thickness effect on curvature. The approximation is accurate to within 1 mu Gal for Bullard caps of less than 4-km thickness. The accuracy of the approximations is illustrated for the parameters of thickness and Bullard radius.