A hydrometer is a floating instrument for determining specific gravities; especially of liquids and solutions. It is usually a hollow glass or metal instrument, weighted at one end so as to float upright. The stem of the instrument is graduated so as to indicate the gravity of the liquid. Many instruments, for use with specific solutions, have arbitrary scales and are usually known by the names of the inventors, such as Balling and Plato. Homebrewers and winemakers use hydrometers to measure the amount of alcohol in their fermented beverages.