Glass Hydrometer: Blown, Weighted with Shot, Calibrated in Known Liquids

A hydrometer is a glass float with a thin stem: it sinks deeper in light liquids and rides higher in dense ones, so a scale on the stem reads specific gravity. Baumé fixed his scale at two points, pure water and 15 % brine (Ure, 1839); here the scale is marked in sugar solutions of known strength, giving degrees Brix, and in liquids weighed for their specific gravity. The picture: an old glass hydrometer with a paper scale inside its stem and a ballast bulb below.