A portable mercury barometer that was invented in 1800 by the French instrument maker Jean Nicholas Fortin (1750–1831). In other mercury barometers air pressure is calculated from the distance between the level of the mercury in the reservoir and in the tube. In the Fortin barometer the bottom of the reservoir is flexible (originally it was made from leather) and can be raised or lowered by means of a screw, allowing the surface of the mercury to be adjusted to a predetermined level. A vernier scale on the side of the barometer tube is then lowered until its base touches the top of the mercury in the reservoir and a reading is taken from the fixed scale marked on the tube. Before travelling, the base of the reservoir is raised using the adjusting screw until both reservoir and tube are filled with mercury. The Fortin barometer makes no correction for changes in temperature, which affect the volume of mercury, or for capillarity, which introduces inaccuracies. The instrument is nevertheless sufficiently accurate for most purposes. See also fiducial point; Kew barometer.
Subjects: Ecology and Conservation.