The barullion, forerunner of the hydrometer, is expressly cited in literary texts of late antiquity, though several traces lead us to believe that this device was known even in an earlier epoch. The appearance of the barullion was motivated by the need to perform precise measurements on the density of liquids and on the specific weight of bodies: this small apparatus presented small variations depending on the uses it was intended for. In the case we present - a hydrometer used to quantify the density of liquids - the device is made up of a glass container, inside which a ballasted float is placed, surmounted by a graduated rod. By attaching a weight to the lower extremity of the float, it will sink differently according to the liquid in which it is immersed. The measurement is thus based on Archimedes' principle whereby the float receives an upward thrust that is directly proportionate to the density of the liquid in which it is immersed. The density is then read in correspondence to the notch on the rod and the level of emergence.