The barometer is an instrument for measuring atmospheric pressure. The pressure indicated can aid in predicting short term weather.

There are two main types of barometer in use, the mercury barometer, which can either be in the form of a "stick" or a "wheel", and the aneroid barometer, a later invention and most commonly available.

Italian Evangelista Torricelli, an associate of Galileo, is generally credited with inventing the mercury barometer in 1643. Galileo suggested to Evangelista Torricelli that he use mercury in his vacuum experiments.

A mercury barometer has a glass tube with a height of at least 84 cm, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury creates a vacuum in the top of the tube. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir.

At the time of its invention, Torricelli was not experimenting with air pressure but with the creation more...of vacuums, and whether or not the air had weight, and it was not until much later that it was realised the changes in air pressure measured by the barometer could be used for short term weather forecasts.

High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir.

The long tube used by Torricelli meant that the barometer was in the form of a long stick, and gave rise to the expression of atmospheric pressure in inches or millimeters or feet.

In 1665, Englishman Robert Hooke created the wheel barometer which added a circular scale and dial assembly to the mercury barometer.

In 1843, the French scientist Lucien Vidie invented the aneroid barometer, which uses a small, flexible metal box called an aneroid cell to measure changes in atmospheric pressure. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives a mechanism so that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer.

The empty box is prevented from collapsing by a strong spring. Aneroid means fluidless, no liquids are used, the metal cell is usually made of phosphor bronze or beryllium copper.

Today's barometers use electronic sensors instead of mercury and metal aneroid cells.

Mercury barometers are no longer manufactured due to the recognition of the harmful effects of mercury, although old mercury barometers an still be refilled. Use of mercury in manufacturing was banned in most western countries in the 1990s or early 2000s. However mercury barometers emit zero or negligible amounts of mercury vapour, and owning one is no threat to your health. In 2006 the European Union excluded barometers from a ban on the use of mercury in manufacture or repair.

The mercury barometer makers were at their peak from about 1830 to 1890. England, due to it's robust economy and lust for science, was the home to most of these makers, though many were also found in France, Germany, Holland, Ireland, Scotland, Belgium, Portugal, and here in America.

There were a number of famous makers based in London, such as Negretti & Zambra, Troughton & Simms, Comitti and Son, and Dolland & Co. and some barometers were made and stamped with the name of the upmarket retailer through whose premises they were sold. Nicholas Goodison in his book English Barometers 1680 - 1860 Antique Collectors Club, 1977, lists almost 2000 makers and retailers of barometers during this period.

Mercury barometers were precision scientific instruments and would have only been owned by the very wealthy. This is reflected by the use of exotic timbers such as rosewood, mahogany and walnut, and the craftsmanship entailed in their manufacture.

The Admiral Fitzroy Barometer is a style of mercury barometer based on work by Admiral Fitzroy in the late 1850's.

Admiral Robert Fitzroy achieved lasting fame as the captain of HMS Beagle during Charles Darwin's famous voyage, and in his position as Meteorological Statist to the Board of Trade in 1854, as a pioneering meteorologist who made accurate weather forecasting a reality.

The Admiral Fitzroy Barometer usually consists of a mercurial barometer (open tube type), thermometer, printed instructions and a storm glass or storm bottle, the latter contains a mixture of chemicals which change their appearance depending on the type of approaching weather.

Aneroid barometers measure the air pressure by means of a vacuum chamber with flexible side. Any change in the air pressure alters the thickness of the chamber, and the movement is measured and linked to an indicator needle through levers and pulleys. They were popular in the mid to late Victorian and Edwardian period, and saw the barometer move from an instrument that could only be owned by the very wealthy to become an affordable aspiration by the rising middle classes.