Hydrometer housed in a wooden box with a series of weights and a wooden thermometric scale (the thermometric tube is missing). Inside the lid is a label with the name of the maker, Barthélemy.

This instrument was used to measure the specific gravity (density) of liquids. It comprises a spherical silver float whose underside is fitted with a small shaft on which the weights can be screwed. A graduated rod is attached to the top side.

The specific gravity of the liquid in which the instrument was partially immersed was determined from the portion of the graduated rod emerging from the liquid itself.