Already Galieo discovered that the density of liquid is changed by varying temperatures. Our thermometers are manufactured on this principle.
The temperature can be read from the lead discs hanging from the coloured spheres. The weight differential of the spheres is only 2/1000 grams and ensures an accuracy of 1°C. The spheres descend lowly when the temperature increases and rise when the temperature falls, and can be read from the lowest lead disc of the high floating spheres.