1750-1831. French physicist and
instrument-maker who invented a mercury
barometer that bears his name. It measures
atmospheric pressure by means of a column of
mercury, formed by filling a tube, closed at
one end, with mercury and upending it in a
reservoir of the metal. At the upper end of
the tube this leaves a gap (Torricellian
vacuum), which changes size with variations
in atmospheric pressure, expressed as the
height of the column of mercury in
millimetres. On this scale, normal
atmospheric pressure is 760 mm of mercury.