hydrometer scales

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:

Balling scale
Baumé scale
Brix scale
Dujardin scale
Gay-Lussac scale
Öchsle scale
Tralles' hydrometer scale
Twaddell hydrometer scale

resources

Bernadette Bensaude-Vincent.
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.