Mercury-in-glas thermometer, 18th century.
© Science Museum / Science & Society Picture Library
This thermometer, in a folding wooden case, has a paper scale graduated from -35 degrees to +100 degrees Celsius. The Celsius scale of heat measurement was invented in 1742 by the Swedish astronomer, Anders Celsius (1701-1744). The Celsius, or Centigrade, scale initially placed 0 degrees as water's boiling point and 100 degrees as its freezing point. In 1743, Jean-Pierre Christin, of the Societe Royale of Lyon, reversed this, making 0 degrees freezing point and 100 degrees boiling point.