In 300 BCE, Archimedes, studying the mass and volume of objects, demonstrated that the amount of water displaced is related to the mass of the submerged object. In the fourth century CE, Synesius and female Greek mathematician Hypatia created the hydrometer which could weigh a liquid regardless of its volume. 1000 years later, Galileo invented the hydrostatic balance which compares the mass of objects in liquid. It showed that objects float more in heavier liquids. During the French revolution, all measuring units were standardized; 1 litre of water weighed 1 kg, and its density was 1 which means that 1000 kg of water fills up 1 cubic meter. This was called volumetric mass density. Density is the volumetric mass difference between 2 materials, and most are compared to that of water.