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How to deal with pressure.

IF you're in the market to buy an antique barometer and want instant proof it still works, take a tip from my Business Manager (Mrs P): pop a clear plastic bag in your pocket next time you visit an auction or collectors' fair.

When you find the one you want, carefully place it inside the plastic bag, gather the top together in your hand and blow it up like you might if you were popping a crisp bag. Then, gently squeeze the sides of the bag.

Barometers work on air pressure.

Squeezing the bag causes the pressure inside to rise, making the barometer's needle move. If it doesn't, it's broken.

Apparently she picked up this little gem from watching Flog it!, proving beyond doubt how she spends her afternoons and, I suppose, that post lunch programmes about antiques and auctions do have their uses.

I, meanwhile, resolved to learn more about barometers. They started life as scientific instruments, using mercury for measuring heights and for experiments with the air pump. However it wasn't long before scientists realised there was a connection between the action of air pressure on mercury and weather forecasting.

The first instrument used to measure atmospheric pressure was the mercury barometer, invented in 1643 by Italian physicist and mathematician Evangelista Torricelli.

The first domestic "quicksilver weather glasses" for the home appeared in 1677 and by about 1720, domestic demand had easily outstripped that of scientists.

Barometers were first made by specialist companies who manufactured scientific instruments. Once interest in them grew, all sorts of people had a go, with varying degrees of success. Probably most successful were clockmakers, but cabinetmakers, opticians and even gilders and picture framers got in on the act.

Notable names to watch for from the first category include the important London makers Daniel Quare and Thomas Tompion, both of whose clocks make fantastic sums at auction.

Other important makers include mathematical instrument maker John Bird (1709-1776) and opticians Peter Dollond (1731-1820) and George Adams (1750-1795) By the end of the 18th century, maritime trade was flourishing. Knowing the likely weather when putting out to sea became increasingly vital.

Navigational and surveying instruments became essential to progress and towns and cities with close links to ports became recognised centres for their manufacture. Another factor that boosted production was an influx of highly qualified Italian instrument makers. They were accomplished folk who had emigrated in search of a better life, and they were adept at glassblowing and cabinet-making. Their expertise was such that by about 1840, a substantial community of Italian barometer makers began to thrive, virtually ousting English makers. Watch for names such as Zanetti, Fratelli, Bianchi, Bolongaro, Casartelli, Ronchetti and Solca.

Narrow stick barometers of a few inches wide and about 3ft 6ins in length dominated the scene from the earliest of days until about 1780 but remained in production until mid-Victorian times. Cases dating from before 1800 were usually narrower than later examples and had a turned cover over the cistern holding the mercury at the base.

Rising out of this was a narrow, vertical glass tube through which the mercury was forced by air pressure. By 1810, the cover was replaced by a hinged box. Reading barometric pressure was simply a case of looking where the column of mercury stopped on a chart, usually inscribed on an ivory vernier scale.

So-called wheel or banjo barometers, named after their shape, have an indicator on a dial, operated by a float rising and falling on the surface of the mercury. As the float moves, it acts on an attached thread and moves the pointer.

The cases of both these types of barometer often matched the design and decoration seen on longcase clocks of the same period. As fashion changed, so clocks changed with it and barometers followed suit.

Cases in the 1760s were usually in mahogany with satinwood inlay featuring a shell or a star. From the mid 1780s, the glass covering the dial was convex. Regency wheel barometers had cases usually veneered in rosewood, sometimes cross-banded in different coloured wood and inlaid with brass or mother-of-pearl.

The mercury barometer was largely superseded by the invention of the aneroid barometer in 1844 (aneroid means without fluid). An aneroid is a flexible metal bellows that is sealed after having some air partially removed from it. Higher atmospheric pressures squeeze the metal bellows, while lower pressures allow it to expand, as the Business Manager's test proves.

Greater accuracy, smaller size, portability and cheapness of production were the reasons for its success. Spill the mercury from a stick or wheel barometer and it is rendered useless, which is why such instruments should always be carried upright. An aneroid barometer has no such problems.

One of the most charming mercury barometers was named after Admiral Robert FitzRoy (1805-1865), the head of the Meteorological Office of the oard of Trade. Fitzroy commanded MS Beagle on the ve-year survey of the outh American coast ith Charles Darwin oard as naturalist. He so wrote a number learned papers on ather forecasting, hich led to his setting a storm warning tem for ships. This came the forerunner egular weatherecasts for the general blic.

The design of the zRoy barometer isily distinguishable m the earlier types. The flat, glazed body ncloses a fully visible, wide-bore mercury tube and cistern, behind which is a usually a porcelain register plate, although cheaper examples were printed on paper, inscribed with scale and markings.

A rising barometer is indicated by a moveable pointer to the left of the register and the falling barometer by a corresponding pointer to the right. When set at the correct times and subsequently read off, the findings can be checked on a printed chart pasted either side of the mercury tube, giving explanations, which Fitzroy had determined, as to the kind of weather to expect from particular barometric situations.

More often than not, a thermometer and a "storm glass" were also fitted. This latter device is glass tube filled with distilled water and chemical crystals whose solubility and position is supposed to predict the weather.

More amusing than accurate, it goes along the lines of: crystals at the bottom of the tube - frost; rising to the top - wind; muddy liquid - rain; clear liquid - sunshine.

gimbal-mounted n rosewood meter and mometer by J. vico & Co, valued at 0-PS1,000: Oak cased heel barometer and hermometer by Negretti & Zambra of London, valued at PS120-PS160; A Victorian stick barometer and hermometer valued at PS300-PS400; late Victorian brass cased neroid barometer nd thermometer by Newton & Co, alued at PS120-PS160 nd an Admiral itzRoy barometer, rca 1880, valued at PS650-PS850 n rosewood meter and mometer by J. vico & Co, valued at 0-PS1,000: Oak cased heel barometer and hermometer by Negretti & Zambra of London, valued at PS120-PS160; A Victorian stick barometer and hermometer valued at PS300-PS400; late Victorian brass cased neroid barometer nd thermometer
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Publication:Western Mail (Cardiff, Wales)
Date:Apr 22, 2015
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