Editing Barometer (section)

==== Fortin barometer ====
[[File:Baro 2.png|thumb|upright|Reservoir of a Fortin barometer]]

[[Jean Nicolas Fortin|Fortin]] barometers use a variable displacement mercury cistern, usually constructed with a thumbscrew pressing on a leather diaphragm bottom (V in the diagram).  This compensates for displacement of mercury in the column with varying pressure.  To use a Fortin barometer, the level of mercury is set to zero by using the thumbscrew to make an ivory pointer (O in the diagram) just touch the surface of the mercury.  The pressure is then read on the column by adjusting the [[vernier scale]] so that the mercury just touches the sightline at Z.  Some models also employ a valve for closing the cistern, enabling the mercury column to be forced to the top of the column for transport.  This prevents water-hammer damage to the column in transit.