A hydrometer is an instrument to measure the density of a liquid. In essence it consists of a closed glass tube calibrated along its stem and blown into a bulb, which is weighted at the end. The hydrometer floats upright in the liquid to be tested: the denser the liquid, the more the instrument is buoyed up (in accordance with Archimedes' principle). The scale is read at the level of the liquid's surface. Though the instrument measures density directly, it is usual to calibrate the stem in terms of specific gravity, the ratio of the density of the liquid to the density of water at that temperature. Other scales are used for specific purposes.