Explanation of why certain substances float on water depends upon what is called the law of buoyancy. When a ship is constructed, it is necessary to lay out the plans in accordance with the principle or law involved. Consequently a knowledge of the law of buoyancy is important.

When a body is immersed in a liquid, it is buoyed up by a force equal to the weight of the liquid displaced. The weight of a floating body is equal to the weight of the liquid displaced. The upward pressure, or buoyancy, of the liquid may be regarded as exerted at the center of gravity of the displaced water, B, which is called the center of pressure or of buoyancy. A vertical line drawn through it is called the axis of buoyancy or of flotation (Fig. 50). In a floating body at rest, a line joining the center of gravity of the body, G, and the center of buoyancy of the water, B, is vertical, and is called the axis of equilibrium. When an external force causes the axis of equilibrium to lean, if a vertical line is drawn upward from the center of buoyancy to this axis, the point where it cuts the axis is called the meta-center. If the metacenter is above the center of gravity the distance between them is called the metacenter height, and the body is then said to be in stable equilibrium, that is, tending to return to its original position, when the external force is removed.

Fig. 50. - Principle of Buoyancy.

Fig. 51. - Principle of Stability.