Hydrometers measure the density of liquids. Generally they are floated in the liquid to be measured; the depth to which they sink indicates the density of the liquid. Many different scales have been developed for hydrometers, often for particular trades. See:
The Chemist's Balance for Fluids.
in Frederic L. Holmes and Trevor H. Levere, editors.
Instruments and Experimentation in the History of Chemistry.
Cambridge: The MIT Press, 2000.
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Last revised: 2 March 2010.