4 inch barometer to help predict incoming weather systems. 7 inch Galileo Thermometer. The thermometer works on the principle discovered by Galileo that density of a liquid changes as tempeature varies. Each of the glass balls is exactly weighted by partially filling the spheres with liquid. When the temprature rises, the liquid in the glass cylinder becomes less dense and the heavier balls sink slowly to the bottom. The lowerst tempertaure ball within the group at the top of the cylinder tells the correct temperature.