Scientists in the 1500s noticed that changes in the volume of a liquid or gas were accompanied by consistent changes in its temperature. To accurately measure these changes they devised thermoscopes, the simplest of which were glass tubes, with one end projecting into a vial of water. Rising temperatures would expand the air in the tube and force the column of water down. Galileo invented a thermoscope in around 1600 using water as the liquid variable. A sealed thermometer with alcohol and degree markings was created by the grand duke of Tuscany in 1641, and in 1664 English scientist Robert Hooke designed a thermometer with zero as the starting point.
A reliable temperature scale had to wait for the work of German physicist Gabriel Fahrenheit, who invented the mercury thermometer in 1714. He also devised the Fahrenheit scale that we use to this day. He set the freezing point of water at 32 degrees and the boiling point at 180 degrees higher (the number of degrees in a semicircle), or 212 at sea level. He also observed that decreasing atmospheric pressure (as at higher elevations) lowers the boiling point.
Calibrating thermometers with 180 degrees between freezing and boiling made sense for many scientific investigations, but it was awkward for others. A scale with exactly 100 degrees—with 0 for freezing and 100 for boiling—was proposed in 1742 by Swedish astronomer Anders Celsius. The new centigrade, or Celsius, scale was adopted as part of the metric system by France in the late 18th century and by the international scientific community in the 1940s.