Areometer.
An instrument for determining the specific gravity of a fluid. It
consists of an elongated body ballasted so as to float vertically and
provided with a mark or a scale. It floats deeper in a light than in a
heavy liquid. If it carries but one mark weights are added until that
mark is reached, when the weights required give the specific gravity. Or
the scale may give the reading directly based upon the depth to which it
sinks. Areometers are often made of glass, ballasted with shot or
mercury enclosed in their bottom bulb as shown. They are used in
regulating battery solutions, and in watching the charging and
discharging of storage batteries.