Interest in atmospheric pressure arose when miners and well-diggers realized that pumps and siphons would only raise water to a maximum distance of about 33 feet (10 m). Hearing that the Grand Duke of Tuscany had a suction pump that could not raise water as far as he wanted, the Italian physicist Evangelista Torricelli (1608-1647) investigated the problem in 1643, creating what is known as the Torricelli tube.
Imagine that you have such a tube of straight glass, 40 inches (100 cm) long, sealed at one end and filled with mercury, and you carefully invert this tube, keeping the open end dipped in a reservoir of mercury. The mercury will retreat down the tube leaving a vacuum at the top. The height of mercury above the reservoir level will be about 30 inches (75 cm), and the weight of the mercury in the tube will be supported by the pressure exerted by the Earth's atmosphere as it presses down on the mercury in the reservoir.
At the time, many natural philosophers were interested in the properties of the vacuum, and the Torricelli tube was a common demonstration experiment at their meetings. Blaise Pascal (1623- 1662) fitted the tube with a graduated scale around the 1660s, and what was a one-off demonstration device developed into an important instrument for measuring the variations of atmospheric pressure.
Pascal asked his brother-in-law to carry a mercury barometer up the nearby Puy-de Dome mountain and learned that atmospheric pressure decreases with height. Edmund Halley quantified the decrease as being exponential. People at the time noted how the pressure changes with the weather, and the barometer, in various forms, has been an essential tool in weather forecasting since the nineteenth century.
