CENTRE OF BUOYANCY. The centre of buoyancy of a floating body is that point within its boundaries corresponding to the centre of gravity of the volume of water or other fluid which is displaced. It is customary to regard buoyancy as a force acting upward and opposed to gravity. This is not in accord ance with fact, for all the phenomena of float ing in water, air or other fluids are due entirely to the attraction of gravity which acts con downward. (See HYDROSTATICS). When a body floats in water it sinks into the water and toward the centre of the earth only to the extent where the earth's attraction for the body and its attraction for the volume of dis placed water are equal at that level. And as the weight of a body is the measure of the earth's attraction for it we say a floating body displaces its own weight of water. A floating body will therefore sink into water only to a point where the water beneath it is attracted toward the earth's centre more forcibly than is the floating body. The same phenomena may be observed in the case of a balloon. The bal loon filled with a gas that is lighter than air at the sea-level will rise to a position where the earth's attraction for the balloon and its at traction for the volume of air displaced by the balloon are exactly equal. The centre of buoy
ancy of the balloon is the centre of gravity of the body of air displaced by the balloon.
When the centre of gravity of a floating body is below its centre of buoyancy (equivalent to its point of support—see EQUILIBRIUM), the body will float in stable equilibrium. When the centre of gravity is above the centre of buoy ancy the equilibrium is unstable, and the body is liable to roll over in the water into such a position that its centre of gravity is below its centre of buoyancy. The same observations hold good as to bodies floating in air. A body which will float in unstable equilibrium in water Would probably be in unstable equilibrium in 'mercury, the volume of mercury displaced hav ing a much higher centre'of 'cavity than the displaced water, and therefore lifting the centre of gravity of the floating body far above its 'centre of babyancy,