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.