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A Galileo thermometer or Galilean thermometer is a thermometer that uses the density of liquids to measure temperature. The thermometer was invented by Galileo Galilei, who had noticed that the density of a liquid changes with temperature. The thermometer is made of a glass container, filled with a liquid. Inside, there are other containers, usually with other liquids. When the temperature changes, so does the density of the "container" liquid. As a consequence, the smaller containers may fall to the floor, or rise to the surface.
Usually, such thermometers are calibrated, to either 2° or 1°. The temperature is then that of the bulb with the lowest temperature, floating above.
Now, Galileo thermometers are used mainly for decoration rather than for measuring temperature, although you can still read the temperature off them.