A hydrometer is based on the relation of buoyancy to density. It is a floating glass body with a cylindrical stem and a bulb filled with a metal weight. Depending on how deep it sinks in the sample, the density can be read directly from a scale on the upper part. The deeper it sinks, the less dense the substance is. If we immersed the hydrometer in a glass of water, it would sink deeper than in a glass of syrup, where it would float higher.