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A Galileo thermometer (or Galilean thermometer) is a thermometer made of a sealed glass cylinder containing a clear liquid and several glass vessels of varying densities. As the temperature changes, the individual floats rise or fall in proportion to their respective density.
It is named after Galileo Galilei because he discovered the principle on which this thermometer is based—that the density of a liquid changes in proportion to its temperature—and he also invented a thermoscope based on this principle.
The Galilean thermometer works on the principle of buoyancy. Buoyancy determines whether objects float or sink in a liquid, and is responsible for the fact that even boats made of steel float in water (while a solid bar of steel sinks).
The only factor that determines whether a large object rises or falls in a particular liquid is the object's density relative to the density of the liquid. If the object is denser than the liquid then it sinks, as it is heavier than the liquid it displaces. If the object is more dense than the liquid then it begins to sink until the weight of the displaced liquid becomes equal to the object's weight; then it floats at that depth.
