The barometer is one of the key instruments in the science of meteorology, and it was instrumental in answering a fundamental question of natural philosophy at the time of its invention: "Does the air have weight?". Construction of the first barometer in the mid-17th Century is commonly attributed to Evangelista Torricelli of the Florentine Academy, although Gasparo Berti unknowingly built the first one while trying to produce a vacuum a few years earlier.

The story behind the development of the barometer goes like this. In a letter dated 1630, Giovan Battista Baliani a patrician from Genoa, asked Galileo Galilei for the reason why water did not rise through the syphon he had designed to carry water over a hill about 21 metres (70 ft) high.

The prevailing thinking on how a suction pump worked held that the pump action created a vacuum, and, since "nature abhors a vacuum," the water immediately rose to fill the evacuated space. There was, it was believed, no limit to the height to which water could be raised. Experience showed something was amiss. Galileo investigated the situation and reported that the working limits of a suction pump made it impossible to raise a column of water any higher than 11 metres (33 ft). Beyond this limit, the vacuum force was insufficiently powerful to prevent the column of water from collapsing.

Galileo passed his concern over the problem to his disciple Torricelli. Torricelli devised an experiment -- conducted by his student Vincenzo Viviani in 1643 -- which actually proved air had weight. To do so, they constructed the prototype mercury barometer. Torricelli had first built a water barometer, but it required a very long (18 m / 60 ft) and clumsy glass tube. By substituting mercury, which at room temperature is a liquid and about thirteen times more dense than water, he was able to reduce the working tube length to around 90 cm (35 inches).

Torricelli's instrument consisted of a long-necked glass tube with a closed bulbous end. The tube was filled with mercury and then inverted into a basin also filled with mercury. Rather than run completely out of the tube, the height of the mercury column fell to a level of about 76 cm (30 inches) and then remained fairly steady, The top of the mercury column was observed to fluctuate by a few percent, in part due to changes in temperature and in part due to what we now know to be fluctuations in atmospheric pressure above the instrument.

Torricelli was convinced by this experiment that what held up the mercury column was the weight or pressure that the air exerted on the mercury in the basin. He also believed that the space above the liquid created by the descent of the mercury in the tube was completely empty, a true and stable vacuum.

As often happens in science, similar lines of thinking and experiments were taking place elsewhere, and perhaps some information was exchanged between scientists. Thus, although Torricelli is universally credited with inventing the barometer, two other noteworthy efforts must be cited.

Historical documentation also suggests Gasparo Berti, an Italian mathematician and astronomer, may have performed similar experiments to Torricelli's a few years earlier while also working on Galileo's problem. Berti apparently constructed a huge lead tube, around 13 m (40 ft) long and attached it to his house. He filled the tube with water, then sealed the top. When the bottom of the tube was opened into a filled pail of water, the water level in the tube dropped then didn't move. Berti claimed that he had produced a vacuum in the tube above the water line, the result he sought, but likely he did not realize he had also just built a huge, and the first, barometer.

French scientist and philosopher Rene Descartes described the design of an experiment on atmospheric pressure determination as early as 1631, but there is no evidence that he built a working barometer at that time.

On the other hand, if one wishes to be picky and require a true barometer to have a scale by which to quantify readings, Descartes appears to have been first to do so. In a 1647 letter to Marin Mersenne, he wrote:so that we may also know if changes of weather and of location make any difference to it, I am sending you a paper scale two and a half feet long, in which the third and fourth inches above two feet are divided into lines; and I am keeping an exactly similar one here, so that we may see whether our observations agree."

In 1648, French mathematician Blaise Pascal (after whom the basic SI unit of pressure, the pascal, is named) put forward a theory that air pressure decreased with altitude above sea level. He enlisted his brother-in-law Florin Perier to carry a barometer up to the peak of the Puy-de-Dome mountain in the Massif Central of France. Perier was astonished to observe a much diminished mercury column height by about 8.6 cm (3.6 inches) when he reached the 1490-m (4888-ft) summit compared to his base reading, confirming Pascal's hypothesis.

For about twenty years thereafter, development of the barometer was slow. Then in 1665, Englishman Robert Hooke created the wheel barometer which added a circular scale and dial assembly to the mercury barometer. Barometer development and refinement then began a century of great progress.

The word barometer to describe the pressure measuring instrument has been attributed to English scientist Robert Boyle who in a 1669 manuscript Continuation of New Experiments described plans for a truly portable barometer.

The concept of the aneroid (meaning "without liquid") barometer in which pressure changes were detected using a sealed bellows was proposed by the eminent mathematician Gottfried Wilhelm Leibniz around 1700 but never constructed. The first working version of the now-common aneroid barometer was built in 1843 by French scientist Lucien Vidie. By removing the need for the delicate mercury column and easy-to-spill liquid, the aneroid barometer was a very portable instrument. Thus, it became as common a meteorological instrument for explorers and field researchers. With its ability to foretell weather changes, the barometer became a common tool for mariners and farmers who could afford to own one. Soon, many homes had a barometer hanging on their wall.

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