I have, for instance, two Galilean thermometers standing on my bookshelf. These are elegant cylinders filled with a clear liquid in which float multiple glass balls filled with liquids of different colours. The amount and type of liquid inside the floating balls determines their buoyancy, which changes with temperature… and thus the position of the floating balls in the cylinder provides indication of ambient temperature.
As thermometers, they’re not terribly accurate of course, since they’re dependent on 1) perfect calibration and 2) the thermal conductivity and responsiveness of the glass and liquids… but the balls do move up and down in the cylinder as the room warms up and cools down.
Cool, huh? And pretty.
This type of instrument was invented by scholars in the 17th C, based on the principles of buoyancy discovered by Galileo. As such, I feel perfectly justified in adopting these for one of my fantasy civilisations one day… maybe even in the one I’m writing now!
Foe a more detailed description of how they work, see Wikipedia (and no doubt plenty of other sites).
Galilean thermometers are my inspiration of the week. What strange objects of interest do you have around your house? Why do they call to you?