Areometer, hydrometer.

Instrument used for measuring specific gravity; see araeometer.

ETYM Hydro-, 1 + -meter: cf. French hydromètre.
An instrument for determining the specific gravity of a liquid or solid; SYN. gravimeter.
Instrument for measuring specific gravity of liquids.
Instrument measuring specific gravity and density of liquids.
In physics, an instrument used to measure the relative density of liquids (the density compared with that of water). A hydrometer consists of a thin glass tube ending in a sphere that leads into a smaller sphere, the latter being weighted so that the hydrometer floats upright, sinking deeper into less dense liquids than into denser liquids. Hydrometers are used in brewing and to test the strength of acid in automobile batteries.
The hydrometer is based on Archimedes' principle.