Educational and Attractive Galileo Thermometers are designed to add character and charm to any home or office environment and make an attractive gift. Everyone can appreciate the simple beauty of colorful orbs floating in a glass cylinder. Developed by Galileo Galilei, this thermometer is based on the law of physics which states: As the temperature rises, the liquid expands. Its specific gravity decreases and the spheres sink. The current temperature is indicated by the lowest floating glass sphere in cylinder. Features: 13" has 5 spheres 18" has 7 spheres Each hand-blown ball differs in weight by only two thousandths of a gram Calibrated to achieve an accuracy of 1/2 degree F The temperature is shown on gold-plated indicator medallions The liquid within is environmentally safe, biodegradable and ozone friendly Highly educational and entertaining A fantastic gift for the person who is hard to buy for Becomes a conversational piece "The specific gravity of liquids change as they expand at different temperatures.