Archimedes' principle of hydrostatics. Illustration of the purity of gold in a crown being measured using a principle of hydrostatics. This principle (named for 3rd-century-BC Ancient Greek mathematician and inventor Archimedes) states that the buoyant force of a body immersed in a fluid will equal the weight of the fluid displaced. Archimedes used this principle to measure the purity of gold in the crown of Hieron II of Syracuse. The crown weighs the same as a standard mass of gold at left, but has been made of gold mixed with a cheaper, less dense metal. The lack of purity of the gold is revealed at right when the weighing takes place in a fluid. Labelled version available.