Rosewood-cased sympiesometer

Background
The “air barometer,” a device for measuring atmospheric pressure based on the compression of air rather than the weight of the atmosphere, was conceived and demonstrated in the 17th century by Robert Hooke, but remained little more than a scientific curiosity until early in the 19th century. In 1818, Alexander Adie of Edinburgh refined and patented the first practical sympiesometer. In place of air in the closed end of the tube of fluid, Adie used hydrogen, and filled the lower part of the tube with colored oil. Adie’s target market was the marine trade, where he had a thriving business in supplying ships’ barometers, but he wanted to be able to provide a more compact and durable instrument, one that could more quickly and accurately have its readings adjusted for temperature. After about 1820, sympiesometers were made by many suppliers of instruments, and also manufactured by Adie with other sellers’ names on them. They are found in all sizes, even down to a pocket model, and are sometimes combined in the same instrument with a marine barometer.

A sympiesometer is read by first taking note of the temperature shown on the thermometer, then moving the pointer of the sliding scale to that reading on the fixed temperature scale on the right side of the instrument’s backplate. The top of the column of colored oil will then match a point on the 27.5 through 31-inch barometric scale next to the tube of oil, which will correspond to readings taken from a standard mercury barometer. The reading is already temperature-corrected, and markers for both temperature and barometric pressure have been set. A rotating disc at the bottom of the instrument records readings for future reference, since the direction of movement of the oil will want to be known, just as it is on a mercury barometer. Most sympiesometers will also have a table of weather indications (Rain, Change, Fair, etc.) on the inches scale, just like a barometer.

This instrument

The sympiesometer shown here is intended for use on a ship, as noted by the restriction in the glass column to prevent surging and the rosewood case, which withstands corrosive sea air better than most cabinet woods. It also has a fastener at the bottom of the case, which allows it to be fixed firmly to the bulkhead. Sympiesometers were used in a stationary installation on shipboard, rather than gimbaled like a mercury barometer. The reservoir in the glass column is fitted with an anti-spill restrictor, and there is a sliding closing device for the mouth of the column. The backplate is engraved and silvered brass, and is marked Improved Sympiesometer, which refers to Adie’s patent on the improved air barometer, with a maker’s serial number. The instrument is of a large size, easily read.

The maker is Bernard Hennessy of Swansea, a major sea port on the south coast of Wales. The Hennessey firm was a known maker of mathematical instruments in Swansea around 1800 (Taylor, pp366), and Bernard Hennessey, “clockmaker trained in London and contractor to the Admiralty," practiced in Swansea from 1848 to 1875 (Peate).

Circa 1850-1860
23.5” High, 3.5” Wide
No. 1552

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