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BAROMETER

BAROMETER. The mercury barometer had its origins in the investigations being made in Italy during the early seventeenth century to discover why it was impossible to build a suction pump to raise water higher than about thirty feet (10 m). Once it was found that the height attainable was related to the density of the liquid, the experimenters exchanged their cumbersome metal tubes filled with water for shorter glass tubes with the heaviest fluid available—mercury—which was mined in Tuscany. The results of numerous experiments undertaken in Rome, Florence, and elsewhere were widely circulated and discussed.

The first apparatus generally accepted as a barometer was that set up in Florence in 1644 by Evangelista Torricelli (1608–1647), a mathematician and physicist. Torricelli filled a glass tube with mercury, sealed it at one end, and inverted it with its open end in a dish of mercury. The level always fell a short way down the tube, then settled at a height of about thirty inches. He concluded correctly that the mercury column was sustained by the weight of the air pressing on the open surface of mercury, and further experiments convinced him that the space above the mercury in the tube was a vacuum. He noted that the level rose and fell with changing temperature, but he was unable to use his apparatus to measure variations in the weight of the atmosphere because he had not foreseen that temperature would affect the level of the mercury.

News of this experiment circulated quickly among European scientists, who hastened to replicate the experiment. Torricelli's conclusions were not universally accepted because some disputed whether the air had weight, while both Aristotle and the Catholic Church denied the possibility of a vacuum. In France, the philosopher René Descartes (1596–1650) seems to have been the first person, probably in 1647, to attach a graduated scale to the tube so that he could record any changes attributable to the weather. At around this time Duke Ferdinand II of Tuscany organized the first short-lived meteorological network among scientists in other Italian cities, gathering observations of pressure, temperature, humidity, wind direction, and state of the sky.

Descartes, the Minim friar Marin Mersenne (1588–1648), an important nexus for scientific communications, and physicist Blaise Pascal (1623–1662) also discussed whether the mercury column would be shorter if the experiment was performed at the top of a mountain where, presumably, the atmosphere weighed less. Around 1648 Pascal's brother-in-law Florin Perier (1605–1672) set up a tube at Clermont, where it stood at 26 inches 3½ lines (the French line was one-twelfth of a French inch), and carried another tube to the summit of the Puy de Dôme, where the mercury stood at 23 inches 2 lines.

By 1648, the barometer was serving the three purposes that it continued to serve thereafter: as an apparatus for testing the laws of physics, as an instrument for measuring altitude, and as a weather monitor and, later, prognosticator. The words baroscope and barometer, meaning 'instrument for measuring weight', first used by Robert Boyle in the early 1660s, were soon adopted into the Latin, French, German, and Italian languages.

THE BAROMETER AS A PHYSICS APPARATUS

Numerous experiments using variations of Torricelli's apparatus were performed by members of the Accademia del Cimento (The Academy of Trial, or Experiment), a group of Florentine virtuosi active from 1657 to 1667, and published in its Saggi di naturali esperienze fatti nell'Accademia del Cimento (Examples of experiments in natural philosophy made by the academy) in 1667. They sought to discover if the space above the mercury was filled with vapor or air diffused through the glass, and what effect different shaped tubes would have if the dish of mercury, or the entire apparatus, was covered. Many of these experiments were inconclusive, the academicians being unable to interpret their findings. With Otto Guericke's invention of the air pump, the barometer served as a means of measuring the strength of the vacuum created for a whole series of related experiments.

A DIVERSITY OF SHAPES

By 1650, Pascal had probably devised the siphon barometer, which consisted simply of a sealed tube with its open end curved up at the bottom. In 1663, Robert Hooke, demonstrator to the Royal Society, devised the "wheel" barometer, in which a float on the open surface of mercury in a siphon tube was connected to a cord running over a pulley to a counterweight; a pointer on the pulley axle rotated on a large dial, amplifying the small daily variations in height. Many variations of form, usually to enhance portability or to amplify the scale, were proposed in the following century, often by people with no understanding of the glassblower's abilities or the problems of filling such tubes without admitting some air. Among the more practical forms, some of which still survive, were folded, conical, and angled tubes, and tubes with two liquids. By about 1670, the barometer had found its way into wealthier homes and various types could be bought in London and Paris.

In June 1668, Robert Boyle described and illustrated his "portable" siphon, fastened to a board on which a graduated scale was marked, the idea being to send examples to distant places, but he admitted the difficulty of filling such a tube. Credit for the first truly portable barometer is disputed: the barometer maker John Patrick (1654–1730) may have invented the method, and he opposed the patent of 1695 filed by the clockmaker Daniel Quare (1648/9–1724). The tube was sealed into a boxwood cistern with a leather base; a movable plate driven by a screw pressed up on the bag until the mercury filled the tube, after which the instrument could be safely transported. Quare saw this as a means of making domestic barometers in London for sale to provincial customers, but this eminently practical device enabled the subsequent development of mountain and marine barometers.

MOUNTAIN AND MARINE BAROMETERS

The first such measurement in England was probably that made in 1653 by Henry Power, a physician of Halifax, Yorkshire, who reported that the mercury reached only 26 inches at the summit of his local hill. Robert Boyle recognized that, as the mercury fell, even when ascending a church steeple, so it would rise if the barometer was taken down into a mine. In 1672, this observation was confirmed by George Sinclair, a Scottish mining surveyor.

In the early days, explorers and surveyors carried their glass tube, bowl, leather bag of mercury, and graduated rule, and assembled the barometer for each observation, a practice that extended into the eighteenth century, when French academicians sought to measure altitudes of the high Andean peaks, the highest mountains then known. The mathematical formula for the relationship between the altitude and height of mercury was difficult to establish, and astronomer Edmund Halley's 1685 proposal was only the first step on a complex path.

Although the portable domestic barometer became available in the late seventeenth century, the Genevan scientist Jean-André de Luc (1727–1818) was the first to design, around 1750, a robust apparatus consisting of a siphon tube, with thermometers and a plumb-bob, neatly packed in a wooden case. A scale was laid alongside both levels of mercury to measure the distance between that in the tube and that in the open arm. After taking the reading, the tube was tilted until mercury filled it; then, by closing an ivory tap in the siphon and draining off the surplus liquid, the instrument could be carried safely to the next station. The Genevan scientist Horace-Bénédict de Saussure (1740–1799) carried a de Luc barometer to the summit of Mont Blanc, Europe's highest mountain, in 1787.

De Luc's siphons were soon replaced by straight-tube barometers fitted with a leather bag and portable screw, the whole being contained in a slender cylindrical case. In the higher mountains so much mercury descended from the tube, raising the level in the cistern, that the scale alongside the tube became inaccurate. Because the level in the cistern was invisible, a float was inserted in the cistern; as its protruding tip rose against a small graduated scale, the true distance between the two levels could be calculated from this reading.

In his Discourse Concerning the Origins and Properties of Wind (1671), Ralph Bohun (1639–1716) called for the use of a barometer to predict hurricanes, particularly at sea. On board a moving ship, however, the mercury oscillated in the tube and, on occasion, struck the top of the tube and broke the glass. Numerous ineffective designs were proposed in France and England before the London instrument maker Edward Nairne (1725–1806) produced a tube whose central section was constricted to one-twentieth of an inch in diameter. This kept the mercury steady. The barometer, suspended in gimbals, performed satisfactorily on James Cook's second voyage of 1772–1775 and provided the model for marine barometers thereafter.

METEOROLOGY

The height of the mercury column was soon recognized as related to changes in the weather, but the first experimenters were surprised that the mercury fell on rainy days, when they supposed that the water-laden atmosphere was heavier. Soon, however, the correlation between high mercury and fine weather, and between falling or low mercury and rain, encouraged makers to add "Fair," "Changeable," and "Storm" to their scales. Because the mercury expanded and contracted with temperature, small thermometers were put on the frame to correct for this effect.

The barograph, or self-recording barometer, made a late appearance on the architect Sir Christopher Wren's somewhat improbable "Weather Clock." Constructed in 1663, it consisted of several instruments, each of which registered by impressions on a paper chart moved by clockwork. Hooke added a barograph prior to 1681; from the description, he appears to have caused the pulley of a wheel barometer to make similar impressions on the chart. In 1765, the clockmaker Alexander Cumming (1733–1814) constructed a large and elegant continuously recording barograph for King George III (ruled 1760–1820); it was a siphon barometer, the float supporting a light frame carrying a pencil that marked a rotating circular chart. Within a few years similar instruments were being made in France.

See also Academies, Learned; Boyle, Robert; Clocks and Watches; Descartes, René; Ferdinand II (Holy Roman Empire); Hooke, Robert; Mersenne, Marin; Pascal, Blaise; Physics; Scientific Instruments; Technology; Weather and Climate; Wren, Christopher.

BIBLIOGRAPHY

Archinard, Margarida. De Luc et la recherche barometrique. Geneva, 1980.

Golinski, Jan. "Barometers of Change: Meteorological Instruments as Machines of Enlightenment." In The Sciences in Enlightened Europe, edited by William Clark, Jan Golinski, and Simon Schaffer (Chapter 3; pp. 69–93). Chicago, 1999.

McConnell, Anita. "Origins of the Marine Barometer." Annals of Science. Forthcoming.

Middleton, W. E. Knowles. The Experimenters: A Study of the Accademia del Cimento. Baltimore and London, 1971.

——. The History of the Barometer. Baltimore, 1964; reprint, Trowbridge, U.K., 1994.

ANITA MCCONNELL