It's a must-have for the desk of any science enthusiast. The Galileo Thermometer is based on a thermoscope invented by Galileo Galilei in the early 1600s. The thermometer consists of a sealed glass tube that is filled with water and a number of floating spheres. The glass spheres are filled with a colored liquid and float up and down according to changes in the temperature. When the temperature rises the spheres will sink and inversely when the temperature falls the spheres will rise due to changes in the specific gravity of liquids. You always read the lowest of the floating temperature tags to determine the correct temperature.