Noun
An instrument for determining the specific gravities of liquids, and thence the strength spirituous liquors, saline solutions, etc.
An instrument, variously constructed, used for measuring the velocity or discharge of water, as in rivers, from reservoirs, etc., and called by various specific names according to its construction or use, as tachometer, rheometer, hydrometer, pendulum, etc.; a current gauge.
Source: Webster's dictionaryFirst the hydrometer is floated in the reference liquid (shown in light blue), and the displacement (the level of the liquid on the stalk) is marked (blue line). Source: Internet
Hydrometer main The relative density of a liquid can be measured using a hydrometer. Source: Internet
From the Archimedes buoyancy principle, the buoyancy force acting on the hydrometer is equal to the weight of liquid displaced. Source: Internet
In accordance with the way in which hydrometers are usually graduated, Δx is here taken to be negative if the displacement line rises on the stalk of the hydrometer, and positive if it falls. Source: Internet
'In his letters he describes a hydroscope (really a hydrometer) he has made as well as a catapult. Source: Internet
It is, of course, necessary that the hydrometer floats in both liquids. Source: Internet