Clairaut derived the formula under the assumption that the body was composed of concentric coaxial spheroidal layers of constant density. This work was subsequently pursued by Laplace, who assumed surfaces of equal density which were nearly spherical. The English mathematician George Stokes showed in 1849 that the theorem applied to any law of density so long as the external surface is a spheroid of equilibrium. A history of more recent developments and more detailed equations for g can be found in Khan.