One of our volunteers, Nicky, mentioned a quote from the Elizabeth Hervey diaries which talks about “the bar lower than ever I saw it before” on November 9th 1800 (D6584/C/95/99) just before a hurricane. We presumed this mentioned a bar on a barometer, leading to a discussion of how barometers actually work.
Barometers measure atmospheric pressure to predict short term changes in weather. Evangelista Torricelli is generally credited with creating the first barometer in 1643, but there is some evidence that one may have been unintentionally created by Gasparo Berti between 1640 and 1643. These first barometers used either water or mercury. Torricelli was trying to resolve the limits of suction pumps used to raise water, so used mercury as it is thirteen times denser than water.
The method was fairly simple, a tube with one closed end was filled with mercury. It was then placed vertically in a basin of mercury and the mercury in the tube sank slightly, creating a vacuum at the top of the tube (right). Atmospheric pressure would then cause the mercury in the tube to rise or fall. All that is then needed is to measure the height of the mercury in the tube! The wooden cased antique mercury barometers work the same way, with a reservoir of mercury at the bottom of the glass tube.
Meanwhile, water based barometers work on a similar principle, with external atmospheric pressure affecting the height of a liquid, but look different. A glass chamber contains the body of water, with a tube starting below the water level open to the atmosphere above the water level (left).
A stick barometer, such as this French one from circa 1800 (above), which has been used since the eighteenth century, had a small bar that would be manually pushed to level with the mercury, to make the barometer easier to read accurately. There have been other barometers, using dials and even ways of measuring pressure without a fluid. However a stick barometer is probably the type of barometer Elizabeth Hervey is referring to, and the low bar indicated imminent stormy weather.
French stick barometer circa 1800 from ArtListings https://www.artlistings.com/Clocks/Barometers/A-French-polychrome-painted-stick-barometer.-circa-1800
All sketches are my own.