When a piece of wood or other material of density less than that of water is placed in water it sinks until the weight of water displaced is just equal to its own weight. It then floats. Normally, a hydrogen balloon ascends in still air at a steady rate, but it can be made to float if the quantity of gas it contains is adjusted so that the average density of the balloon is just equal to the density of the surrounding air. The weight of the displaced air will then be equal to the weight of the balloon. The above two examples illustrate the law of flotation. A floating body displaces its own weight of the fluid in which it floats. It must be understood, however, that this does not express a new principle, but is merely an expression of Archimedes‘ principle applied to a special case.