Galileo Thermometer

The Galileo Thermometer works because of changes in liquid density as temperature changes. It can lag behind a mercury thermometer, but the lowest floating glass orb in the tube should be marked with the approximate current room temperature:

This 2.5 minute video includes a description too:

This devices bears Galileo's name because he is credited with discovering the underlying principle, that liquid density changes as a function of temperature. Galileo actually did not invent, it but did invent a similar device called a Thermoscope:

More background info is available on "The Thermometer" page of the Galileo Project from Rice University.