Buoyancy is a physical law that was discovered by the Greek mathematician and inventor Archimedes in the third century BC. The principle, called Archimedes' principle, states that any body floating or submerged in a fluid (gas or liquid) is buoyed upward by a force equal to the weight of the displaced fluid. Balloons rise because the air pressure acting upward against it is greater than the pressure above that is pushing downward. The buoyancy is numerically equal to the weight of the fluid (air in the case of a balloon) displaced. An object surrounded by air (like a balloon) is buoyed upward by a force equal to the weight of the air displaced. If the mass of the object is less than the weight of an equal volume of air, the object rises. If the weight of the object is greater than the mass of an equal volume of air, it falls.