Hydraulic scale allows to measure body weight with a system of scaled communicating vessels.
A tube filled with liquid is connected to the platform you stand on. According to Pascal’s law, while your body weight presses on the platform, it increases pressure inside the tube. This pressure, known as hydrostatic pressure, depends on liquid density, standard gravity and height of the liquid column. Density and standard gravity do not change during this experiment, so increase of the pressure is expressed by the rising level of the liquid inside the tube.
Stand on the platform. Check the liquid level in the tube. Height of the liquid column featured on the scale should match your weight in kilograms.