A Galileo thermometer

The colourful glass bulbs inside the glass cylinder indicate  temperature of the room. When the temperature of the liquid changes, its density changes as well. The bulbs, floating in the liquid, fall down when the temperature rises. Which of them float and which fall, depends in liquid’s density.

The glass bulbs are filled with colourful liquid and equilibrated to float by small weights. The mass of the bulbs is matched, at a given temperature, with Archimedes’ force coming from the liquid in the cylinder. If the temperature in the room rises, the liquid in the cylinder (e.g. alcohol) expands and its density becomes smaller. The Archimedes  force acting on the bulbs decreases because their volume does not change as dramatically as the volume of the liquid in the cylinder does and as a result some of the bulbs fall down.  

The problem may arise when the temperature is either too high or too low, as, subsequently all of the bulbs will either fall to the bottom or float at the surface.