On the Equilibrium of Planes (Ancient Greek: Περὶ ἐπιπέδων ἱσορροπιῶν, romanized: perí epípedon isorropión) is a treatise by Archimedes in two volumes. The first book contains a proof of the law of the lever and culminates with propositions on the centre of gravity of the triangle and the trapezium.[1][2] The second book, which contains ten propositions, examines the centres of gravity of parabolic segments.[1]