Evangelista Torricelli became court scientist to the Duke of Tuscany in 1642. He experimented with fountains in Florence and a long tube over ten metres (33 feet) high filled with water that was in his house. In 1644 Torricelli described how he took a glass tube about one metre (3.3 feet) long, sealed at one end, and completely filled it with mercury, which is much denser than water. Then, holding his finger over the open end of the tube, he inverted it under the mercury contained in a large bowl and removed his finger. The mercury fell to a height of about 76 centimetres (30 inches) above the mercury level in the bowl and stayed there. This is still called Torricelli’s experiment and the space at the top of the tube above the mercury in barometers is known as the Torricellian vacuum.
Answered by Alison Morrison-Low, National Museums Scotland.