The variable

History

Mining engineers in Italy in the seventeenth century, finding it impossible to draw water by single-stage suction-pump to a height of more than about 10 metres, sought an explanation; this also tied in with the question posed by Aristotle as to whether a vacuum could exist in nature. In 1643, Torricelli performed his classic experiment with mercury in an inverted tube and showed that the reason for the pumping problem was that a vacuum formed over the water if suction continued to be applied beyond the height of 10 metres. The atmosphere has weight, and it is the pressure of this weight that supports the liquid column to a height such that its pressure equals atmospheric pressure. The word barometer was coined from the Greek root baros, meaning weight. Five years later Pascal and Perrier showed that the pressure was less at the top of a mountain than at its base, and it was soon realised that barometric pressure also varied with the weather.

At the same time, Robert Boyle was studying in Italy, reading Galileo's writings (which included references to air pressure), and he became aware of the Torricelli experiment. When he returned to England he brought the idea of the barometer with him, and went on to formulate his theories about the relationship between the pressure, temperature and volume of gases – Boyle's laws.