A hydrometer is an instrument used to measure the density or specific gravity of liquids base on the principle of buoyancy. It typically consists of a sealed glass tube with a weighted bottom that floats in the liquid being measured. Hydrometers are commonly used in various applications, including brewing, winemaking, and measuring salinity in aquariums. to use a hydrometer, you simply place it in the liquid, and the level at which it floats indicated the specific gravity or density of the liquid.