Liquids are sometimes used in measuring devices. A thermometer often uses the thermal expansion of liquids, such as mercury, combined with their ability to flow to indicate temperature. A manometer uses the weight of the liquid to indicate air pressure.
The free surface of a rotating liquid forms a circular paraboloid and can therefore be used as a telescope. These are known as liquid-mirror telescopes. They are significantly cheaper than conventional telescopes, but can only point straight upward (zenith telescope). A common choice for the liquid is mercury.