Early barometers are extremely rare, highly sought-after and valuable, while later models, though less valuable, can be worth hundreds of pounds.

Mr Jonathan Hills, head of the clocks and barometer department at Sotheby's, which will hold a barometer auction on July 22nd, says an early 18th century barometer can cost from £5,000 sterling (€7,616.15) upwards. "And they can go to £20,000 and £30,000, because they are early and they're rare." Barometers are relatively new instruments. In 1643, Evangelista Torricelli realised that air would support a column of mercury. Shortly afterwards, Robert Boyle adapted this discovery to practical use. But it wasn't until the end of the 17th century that a usable barometer was produced, says Mr Hills. The first kind was the stick barometer, with the scale at the top and a tube of mercury at the bottom. The wheel or banjo barometer was invented in the second half of the 18th century while the aneroid barometer, which uses a metal bellows with a vacuum inside it rather than mercury, was invented in the 1840s. Barometers from the late 18th century tend to be worth between £2,000 and £5,000, he says. "Once mass production started in the 1830s, the price comes down. You can buy a barometer from, say, 1830 to 1850 for £300 to £400. They continued to produce them into this century but not as wonderful pieces of furniture any longer. They tended to be more practical pieces."

Early aneroid barometers from the 1850s tend to be about eight inches in diameter and are valued at about £300 to £500. "But in the main if you're talking about the 19th century, it's the decorative nature of the case which will determine the price," he says.

By the end of the 19th century, some aneroid barometers were as small as two-and-a-half inches in diameter and these are worth from £100, he says.

If you have an antique barometer, check if it has a name on it, although the name could be that of the retailer rather than the maker. Edwin Banfield's book Barometer Makers and Retailers from 1660 to 1900 provides a comprehensive list. "I use it as a bible," says Mr Hills. While it does not give values, it identifies a date by indicating when someone was operating.

The wood used in the barometer can also indicate its date. For instance, walnut could indicate the late 17th century to about 1740. Thereafter, "mahogany came into general use" and barometers "followed the forms and style of furniture of the period," he says.

The date really is "all-important. If it's pre-1800, the value starts going up. Post that, it comes down quite drastically."

Barometers are dated by the name and style. "But of course the styles were copied again at the end of the 19th century," so just because it's walnut doesn't mean it's necessarily early 18th century, he warns. "They used walnut in the 19th century as well." According to Dublin-based Mr Edmund Ryan, who repairs 50 antique barometers a year: "The person who would have a grandfather clock would have an antique barometer." He says that in Ireland stick barometers tend to be worth between £1,000 (€1,269.74) and £3,000, although an exceptional one he knows was valued at £9,000. Banjo barometers tend to be valued between £500 and £1,000, while aneroid barometers tend to cost between £250 and £500.

Mr Edmund Ryan can be contacted at 67 Monastery Drive, Clondalkin, Dublin 22. Telephone: 01 459 1432.

Mr Jonathan Hills of Sotheby's offers free valuations of barometers (readers can send a photograph). In the first instance, telephone: 0044 1403 833 550.