Galileo thermometer

Suspended in the liquid are a number of weights. Commonly those weights are themselves attached to sealed glass bulbs containing colored liquid for aesthetic effect. As the liquid in the cylinder changes temperature, its density changes and the bulbs are free to move ? rising or falling to reach a position where their density is either equal to that of the surrounding liquid or where they are brought to a halt by other bulbs. If the bulbs differ in density by a very small amount and are ordered such that the least dense is at the top and densest at the bottom, they can form a temperature scale.
The temperature is typically read from an engraved metal disc on each bulb. Usually a gap separates the top bulbs from the bottom bulbs and then the temperature would be between the tag readings on either side of the gap. If a bulb is free-floating in the gap, then its tag reading would be closest to the ambient temperature.

Science lesson for the day!