Galileo Thermometer

At least so-called. Although apparently he only discovered the principle that underpins the operation of the device - that the density of a liquid varies with temperature - rather than making the first one. He did make an 'air thermometer', or mare accurately a 'thermoscope' - a device that shows changes in temperature. What makes it a 'thermometer' is the addition of a numerical scale. In this case the number is indicated by the lowest glass ball above the gap. As the liquid in the tube (not water, but some oil that changes density more rapidly with changes in temperature) heats up and cools down, its density changes, meaning that the different weighted balls rise and fall accordingly. The colder the liquid, the denser it is and therefore the more balls will be at the top of the tube. The people that actually designed this sort of thermometer were group of academics and technicians known as the Accademia del Cimento of Florence, who included Galileo's pupil, Torricelli, the inventor of the barometer. Quite why he isn't also credited with this device I don't know. They became popular again in the 1990s when they were made for sale in museums and other shops. Perhaps they thought Galileo was a more recognisable name to help sell them.