On the Equilibrium of Planes II shares the same subject area as the first book but was written at a later date. It contains ten propositions regarding the centre of gravity of parabolic segments exclusively, examining these segments by substituting them with rectangles of equal area. This exchange was made possible by results obtained in Quadrature of the Parabola, a treatise believed to have been published after book one of On the Equilibrium of Planes.[1][2]