Its named after Galileo because he discovered the principle on which this thermometer is based – that the density of a liquid changes in proportion to its temperature. As the temperature increases, the density of the clear liquid decreases causing the bulbs to sink; and vice versa. Tags attached to the bulbs then allow you to determine the temperature. The Galileo Thermometer is an elegant temperature reading device that every science geek needs. It is a sealed glass cylinder that contains clear liquid and glass bulbs that are all different density. As the temperature changes, the balls rise and fall, giving you a temperature reading. The temperature is read from the engraved metal discs that hang off each bulb – if a bulb is floating in the middle, that is the current temperature, or if some are at the top and some at the bottom the temperature will between the bulbs on either side of the gap.