The invention of the barometer, a device to measure air pressure, was a long and arduous process. It required the creation and understanding of a vacuum. Unfortunately one notable philosopher had said a vacuum couldn't exist, and that stalled advancement for centuries. When a physicist accidentally succeeded in creating a barometer, he didn't realize what he had invented! Eventually a third scientist built a barometer and put it to proper use, although he didn't give it its name. Aristotle (384 b.c.-322 b.c.) was the philosopher who said a vacuum could not exist. He correctly believed that the atmosphere had weight, but had no method of measuring it. Galileo disproved a number of Aristotle's claims. In 1638 he published Two New Sciences, in which he stated his belief that a vacuum could exist, but discounted the idea that air had weight and could exert a pressure. About two years later, physicist Gasparo Berti attached a long lead pipe up the side of his house. At the bottom he placed a vessel partially filled with water; at the top, a sealed glass vessel filled with water. Berti opened a valve at the bottom of his pipe and a portion of the water drained out. The evacuated glass vessel at the top now contained a vacuum and Berti was able to experiment with striking the bell. His sole purpose was to discover if sound carried in a vacuum. In the process he had essentially created a water barometer, the water level of which would fluctuate with atmospheric pressure. Surprisingly, neither he nor his colleagues realized what he had invented! Meanwhile, Galileo's former assistant, Evangelista Torricelli, decided to undertake additional experiments following his master's death. In 1643, he filled a tube with mercury, upended it in a bowl filled with mercury, and noticed only a portion of the tube emptied. This was the first barometer. He correctly surmised that the column of mercury was being supported by the atmosphere which was pressing down on the open bowl. Torricelli noted small changes in the level of the mercury from day to day and realized that these deviations represented changes in the pressure of the air, though he never built a permanent instrument to measure these changes. Mathematician Blaise Pascal (1623-1662) followed along in Torricelli's footsteps. He reasoned that if the atmosphere had weight it should decrease as the altitude increased. The best way to find out if this were so would be to make measurements at various altitudes on a mountain.
Pascal was a sickly individual and not up to mountain-climbing, so he convinced his strong brother-in-law to carry two barometers one mile (1.5 kilometers) up a mountain in 1648 and make measurements--not once, but five times! His theory was proven correct. Otto von Guericke's claim to fame was the invention of an air pump in 1647, by which he was able to create a vacuum. He gave considerable thought to air pressure and density and, in 1672, built a water barometer. Guericke, the P. T. Barnum of the seventeenth century, made a brass tube over 34 feet (10m) high, attached a glass ball at the top, and filled it with water. Within the ball was a little figure of a man which floated high in the ball during good weather; in bad weather the water level dropped and the figure floated lower. In 1660, Guericke became the first to use a barometer to forecast weather. Robert Boyle improved on Guericke'sair pump and duplicated Torricelli's experiments. It was Boyle who gave the barometer its name in 1665. Boyle and Pascal independently suggested that a portable barometer could be created by bending the bottom of the tube 180 degrees, thus creating a siphon barometer and eliminating the need for a bowl to contain the mercury.
About this time, improvements in barometers came fast and furious. In 1664 Robert Hooke, Boyle's assistant, designed a wheel barometer that used a mechanical linkage to magnify the movement of the mercury, which was an improvement on a device made by Christopher Wren (1632-1723). French physicist Guillaume Amontons also made improvements to numerous instruments; he designed a barometer that did not use mercury and could be used on board a ship at sea. Several different types of barometers have been invented since then. A typical mercury barometer is made of a glass tube more than 30 inches (760mm) long. ("One atmosphere" holds mercury at about the 30 inch level, hence the need for a tube greater than that length.) The Fortin barometer, invented by Jean Fortin (1750-1831) in the nineteenth century, uses an adjusting screw and flexible leather bag to raise or lower the mercury level. The English Kew barometer, often used on ships, uses a simple mercury cistern, a contracted scale, and a restriction in the tube to lessen oscillations caused by the sea. Italian scientist Lucius Vidi invented the aneroid barometer in 1843. This ingenious device uses no liquid. Instead, a bellows expands and contracts with changes in the atmosphere. The movement is amplified with a rack and pinion arrangement that moves a pointer on a dial. It can also be used as an altimeter. Another version, called an aneroid barograph, traces a continuous record of air pressure changes on paper.
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