Decorative as well as useful, the Galileo thermometer is based on an invention called the thermoscope invented by Galileo Galilei in the 1600s. The downward pull of gravity is essentially what makes the Galileo thermometer work. The expanding or contracting fluid determines which weighted glass balls float, and which sink.
Make sure the Galileo thermometer is in the right place. Keep it away from windows, direct sunlight, heat sources or air conditioning vents.
As the ambient temperature rises, the glass balls will sink to the bottom of the thermometer. As the temperature falls, the balls will wise to toward the surface.
Read the gold tag on the lowest of the floating balls in the upper part of the glass cylinder. This one indicates the current temperature. The tags on each ball are actually weights that determine and calibrate the appropriate weight for each glass ball. The density of each glass ball is slightly different when the calibrated tags are added.
If all the glass balls are at the bottom of the cylinder, the Galileo thermometer is too warm and needs to be moved to a different spot. If the balls are all floating at the top, then the thermometer is too cold. The glass bulbs should be distributed between the top and the bottom of the cylinder.
Glass Galileo thermometers tend to be the most accurate, though plastic Galileo thermometers are available. Keep the glass cylinder clean by wiping with a soft cloth.