pycnometer

noun

1. A vessel of precisely known volume used by weighing to determine the specific gravity or relative density of a liquid.

   “As Galileo showed in his tract La Bilancetta, or “The Little Balance,” a scientist of Archimedes’ stature could have achieved a far more precise result using his own law of buoyancy and an accurate scale, something far more common in the ancient world than a very precise pycnometer, which is used to measure displacement.”