A Galileo thermometer is a sealed glass tube consisting of water and colored glass spheres filled with liquid and connected to metal tags. Each glass sphere and metal tag possesses a different density and represents a certain temperature.
The glass spheres are hand-blown glass calibrated to a certain weight by adding a certain amount of fluid to them. This fluid may be water with food coloring, or it may contain alcohol. The metal tags attached to each glass sphere are calibrated counterweights, and engraved on each tag is a number and degree symbol. Each glass sphere and metal tag combo differs in density from the others in the sealed glass tube.
As the temperature outside of the sealed glass tube changes, the temperature of the water within the tube also changes. As water warms, it expands, and as it cools, it contracts. Both of these actions cause each glass sphere to float or sink. The glass sphere closest to the bottom of the tube, or the one that sinks the most, represents the current temperature of the area where the thermometer is placed.
The Galileo thermometer is based off of the design of Galileo Galilei's thermoscope invention, or a device that moves water up and down in a tube using changes in air density.