Leonardo da Vinci designed an early hydrometer, a device using a floating body and graduated scale to compare liquid density. His notes connect the instrument with experiments on water and other fluids.
A hydrometer floats higher in a dense liquid and sinks deeper in a less dense one. By reading the position of the stem or pointer, a user can estimate relative density. Leonardo’s proposal belongs to his broader investigations of buoyancy, balance and hydraulics.
The surviving sketches do not establish that Leonardo produced a standardized instrument for routine measurement. Later scientists refined hydrometers, and modern versions are used for beverages, batteries, antifreeze and industrial solutions.
This device is easy to confuse with a barometer, which measures air pressure, or a thermometer, which measures temperature. Leonardo studied all three areas of natural philosophy, but only the hydrometer relies on flotation to indicate liquid density.