Vincenzo Viviani relates that, in 1597, in Padua, Galileo developed an instrument to measure changes in temperature called a thermoscope.

The instrument, both simple and ingenious, was the precursor of the modern thermometer.

The Galilean thermoscope comprises a small glass jar fitted with a very thin tube about 50 centimeters long.

You warm the jar in your hands and immerse it upside down in a vessel filled with water. When the jar loses the warmth from your hands, you observe that the water rises in the tube.

The experiment demonstrates the changes in air density caused by changes in temperature.

When the jar is warmed again, the air inside expands, lowering the level of water in the tube. Conversely, when the air cools, its volume shrinks, enabling the water to rise back from the lower vessel into the tube of the jar.