Art meets science in this colorful glass thermometer. In 1593, Italian physicist Galileo invented a thermometer based on the principles of relative density. Individually calibrated weights float in a sealed tube with a mixture of water, ethanol, and hydratreated petroleum where they rise and fall with the temperature and the resulting changes in water density.

Displays temperatures in Fahrenheit in 2° increments from 68° to 80°. Accurate to within 2°. 21"H. Thermometer contents: Normal paraffin.

In the 2nd century B.C., Greek scholar Archimedes discovered the law that states any object in a fluid displaces its own weight of fluid. He went a step further when he described the principle of buoyancy—that if an object is more dense than an equal volume of water, it sinks; if it's less dense, it floats. A simplified way of understanding density in this example is that it's the relative "heaviness" of objects of the same volume—a marshmallow is less dense than a marble of the same size and shape.

Almost 2,000 years later, Italian scientist Galileo discovered that a liquid's density is affected by changes in temperature. As temperatures rise, a liquid's density decreases. This means that objects that float in room temperature water would sink as the water is heated. He applied his discovery to create what is now known as a Galileo thermometer.

In a Galileo thermometer, sealed glass bubbles filled with colored water have weights attached that are marked with different degrees Fahrenheit. The bubbles are suspended in a tube of clear water. The weights give the bubbles different densities from each other and from the water in the tube.

When the atmospheric temperature rises, the density of water in the tube decreases, which changes the relative density of the glass bubbles. So, as the temperature increases, the water's density decreases, which means more bubbles will sink because they are now more dense than the surrounding water. At the highest temperatures, the lightest (least dense) bubble will finally sink. When temperatures drop, the opposite happens and more bubbles rise.

To read a Galileo thermometer, look at the lowest hanging bubble in the middle of the tube—the one that is not entirely sunk, nor entirely floating at the top. This bubble is in equilibrium with the surrounding water, and its tag will tell the current temperature.