It is used to determine the density of a liquid in a direct way.
It is usually made of glass and it consists of a sealed ampoule of glass, with a ballast at its bottom and a hollow cylindrical tube, which has a graduated scale, at its top. The arrangement of the scale varies depending on how dense the liquid is, denser or less dense than water.
The division of the scale which coincides with the surface of the liquid shows us the density ot it.
It is considered to be invented by Hypatia of Alexandria.