Many inventions happen by accident. So it was with the mercury barometer which Evangelista Torricelli, a physicist and mathematician from Florence, Italy in 1641 originally intended to measure elevation rather than predict the weather.

Torricelli’s colleague, Vincenzo Viviani, actually performed the experiment which demonstrated the existence of atmospheric pressure the following year. He took a glass tube, about two arms long, sealed at one end and filled it with mercury. Then, with a finger over the open end, he inverted the tube and plunged it into a bowl of mercury. As soon as he removed his finger, the level of the mercury in the tube dropped a little, leaving a column of mercury that was about 70 cms high—as long as the open end of the tube remained immersed in the bowl of mercury. Torricelli drew the correct conclusion: atmospheric pressure on the surface of the mercury prevented all the mercury from escaping. He also concluded that the space at the top of the tube, caused by the dropping of the mercury, had to be a vacuum. Torricelli concluded that because the height of the mercury varied from day to day, the fluctuations had to be the result of changes in the atmospheric pressure.

In 1648, physicist Rend Descartes and mathematician Blaise Pascal of France added a paper scale to “Torricelli's tube,” creating the first working barometer. It was Pascal's theory that air pressure decreased with altitude above sea level. He enlisted his brother-in-law, Florin Perier, to carry the barometer up to the peak of Mount Puy-de-Dome. Pascal's hypothesis proved correct since Perier observed diminishing mercury column height as he climbed to the mountain's summit.

Then not much happened with the development of the barometer until 1660, when two English scientists, Robert Boyle and Robert Hooke, first observed correlations between variations in atmospheric pressure and changes in the weather. Their experiments led to the development of a practical barometer, and by 1670, the leading clock and instrument makers in England began to construct barometers for scientific and domestic use.

By the time Charles II left the throne of England in 1685, the English dominated barometer production and continued to do so for the next two centuries. Today, the largest quantity and widest range in styles of mercury barometers available to collectors are of English origin.

Early Barometer Makers
Members of the Clockmakers' Company, a guild founded in London under Royal Charter of King Charles I, made most of the barometers during the late 17th to early 18th centuries. Two of the primary makers were Daniel Quare, watch and clockmaker to George I and inventor of the repeating watch, and Thomas Tompion. From 1720 to 1800, London instrument makers, including Jesse Ramsden, acknowledged as the most skillful designer of mathematical, astronomical and navigational instruments in the 18th century, and Peter Dollond, a respected maker of optical and surveying instruments, also made barometers. .

Until the last two of decades of the 18th century, the barometer remained mainly a scientific instrument, available only to serious scientists and the wealthy. Around this time, enterprising Italian migrants began to arrive in England. They made domestic barometers and sold them at affordable prices. From the end of the 18th century, the domestic barometer became one of the “must-haves,” of the upper classes—a thing of science, function and beauty to complement their decor and furnishings and to impress their friends.

The Stick Barometer
The earliest domestic barometer was made in the stick style. The case consisted of a shaft of wood that protected the fine-bore mercury tube. The top few inches, however, housed the scale—printed on paper or engraved on a rectangular plate of silvered copper—that allowed the owner to read the height of the mercury which fluctuated between 28 to 31inches according to the air pressure. Because the mercury might only move by a small fraction of an inch, it was not easy to take a really accurate reading, so barometer makers added a sliding secondary scale, a vernier, to make it possible to observe fluctuations of 1/100s of an inch.

The scale was often protected by a hinged and glazed door and topped by a pediment. Almost the whole length of the tube might be visible, or just those top few inches. The reservoir, or cistern, at the bottom, however, would always be hidden and protected by a cistern cover.

Outline stringing, the choice of quality mahogany veneers often laid in a chevron pattern, and elegant engraving, could all be used to make the barometer not only aesthetically pleasing but functional.

Instruments made for royalty and rich aristocrats, however, were unique and of the highest quality imaginable. The makers were eager to please their wealthy clientele with fine cabinetry and metalwork. Unfortunately, many of the cases suffered when people brought them to America. The atmosphere in England is notoriously wet, so barometer cases fell apart or cracked when they were exposed to America’s drier climate.

The Banjo or Wheel Barometer
During the last quarter of the 18th century, the shape of barometers changed with the arrival of Italian instrument makers to England. They produced a variety of styles in great quantity, marked with their own names. One of the earliest and most prolific families was the Somalvicos, who set up shop in London around 1780, and continued to practice well into the 19th century.

The banjo barometer, also referred to as a wheel or dial barometer, was first made in England around 1790 and soon became the style of choice. As time evolved, additions to the case included a hygrometer, an instrument that measures humidity, a thermometer, a convex `butler's' mirror and a level.

A wheel barometer has a long siphon tube, which makes a U-turn at the bottom, creating a short hook that acts as the “reservoir” for the mercury in what’s commonly referred to as the “short limb.” Air pressure on the surface of the mercury in the short limb is enough to support the whole column. A glass weight rests on the surface of the mercury in the short limb of this tube. Mercury is so dense that the glass weight really does float on it. The maker then sealed the top of the tube to create a vacuum.

Styles of Wheel Barometer
While the technical parts wheel barometers are the same, the cases followed the furniture trends of the time, especially in the type of wood used. Until 1720, walnut was the wood of choice. Later examples included rosewood and mahogany, with ebony and satinwood trim, and inlaid mother-of-pearl or shell also being used. Case styles reflected those of the leading furniture designers of the times—Chippendale, Hepplewhite, Sheraton, and later the revival styles of the Victorian Era. Until 1840, makers usually used brass for the register plates, but in Victorian times, ivory and bone were popular.

Wheel barometers tend to be described by their pediments, diameter of dial, and the number of components they contain. Generally, they had two types of pediments. Round top pediments, popular during the late 18th and early 19th centuries, were just a smooth rounded end to the case. Broken arch or triangular pediment consisted of a molded isosceles triangle, with the apex section removed. The rectangular gap was often ornamented by a brass finial. Though the most popular pediment of the Regency period, by 1830 the scroll pediment had replaced it.

Barometers made in America were scarce until 1840. The ones that did exist were machine-made stick forms, similar to the mantel and shelf clocks made in the same era. These were cheaply made, using grain painting, thin veneers, machine-made moldings and crude assembly methods. Thus, the cost of restoration of these barometers may far exceed their value.

Since barometers are scientific instruments, the country of origin must appear on them, and in many cases, the maker also includes his name and address. Unless specifically made for export, the weather indicators are also written in the language of the country for which the barometer had been made.

The fragile nature of the barometer and their operating mechanisms means that restoration is very common and one that can have a major effect on the value if overdone, or improperly done.

The biggest problem with mercury barometers is that their owners don’t know how to read them properly. Often a barometer will read “Fair” when its raining outdoors. That's because a barometer is a device for predicting weather changes. It was never meant to provide an instant readout of current conditions. A change in the level of the mercury indicates the passage of a high- or low-pressure front over an area and a corresponding change in weather. Falling atmospheric pressure precedes stormy or unsettled conditions while a rise indicates the approach of fair weather.

Mercury barometers should never be moved suddenly because air bubbles in the column upset the calibration of the instrument. Also, because mercury is a hazardous material, mercury barometers cannot be transported by airplane.

Moving Mercury Barometers
Mercury barometers should be moved with special care and kept upright in transit. If the mercury moves suddenly its weight could shatter the glass tube. Stick barometers have a little key square at the base. Turn this until the column of mercury reaches the top of the tube. Wheel barometers must be corked by a specialist before being moved.