The first half of book one deals with the properties of the balance and the law of the lever, while the second half focuses on the centre of gravity of basic plane figures. The law makes use in particular of the first postulate, which states that "equal weights at equal distances are in equilibrium". In Propositions 4 and 5, Archimedes expands on this postulate by proving that the centre of gravity of any system consisting of an even number of equal weights, equally distributed, will be located at the midpoint between the two centre weights.