A body submerged in a fluid experiences a buoyant force equal to the weight of the displaced fluid. Let WA be the weight of the aluminum in air, WW be the weight of the aluminum when submerged, and w be the weight of an equal volume of water.The ratio of the density (D) of the aluminum to the density (d) of the water is equal to the weight (W) of the aluminum divided by the weight (w) of the water displaced. One measures w by measuring WA-WW and INVOKING the principle, or by measuring the displaced volume of water, using the known density of water, and PROVING the principle. The density of the aluminum is then given by:
D = d[WA/(WA-WW)]