When was pressure discovered

Atmospheric pressure is an indicator of weather. Changes in the atmosphere, including changes in air pressure, affect the weather. Meteorologist s use barometers to predict short-term changes in the weather. A rapid drop in atmospheric pressure means that a low-pressure system is arriving.

Low-pressure systems are associated with cloud y, rain y, or wind y weather. A rapid increase in atmospheric pressure pushes that cloudy and rainy weather out, clearing the skies and bringing in cool, dry air. A barometer measures atmospheric pressure in units of measurement called atmospheres or bars.

An atmosphere atm is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius 59 degrees Fahrenheit. The number of atmospheres drops as altitude increases because the density of air is lower and exert s less pressure. As altitude decrease s, the density of air increases, as does the number of atmospheres. Barometers have to be adjust ed for changes in altitude in order to make accurate atmospheric pressure readings.

The mercury barometer is the oldest type of barometer, invent ed by the Italian physicist Evangelista Torricelli in Torricelli conduct ed his first barometric experiments using a tube of water. Water is relatively light in weight, so a very tall tube with a large amount of water had to be used in order to compensate for the heavier weight of atmospheric pressure. In order to keep his experiments more secretive, Torricelli deduce d that he could create a much smaller barometer using mercury , a silvery liquid that weighs 14 times as much as water.

A mercury barometer has a glass tube that is closed at the top and open at the bottom. At the bottom of the tube is a pool of mercury. The mercury sits in a circular, shallow dish surrounding the tube.

The mercury in the tube will adjust itself to match the atmospheric pressure above the dish. As the pressure increases, it forces the mercury up the tube. The tube is marked with a series of measurements that track the number of atmospheres or bars. Observer s can tell what the air pressure is by looking at where the mercury stops in the barometer. In , the French scientist Lucien Vidi invented the aneroid barometer. An aneroid barometer has a sealed metal chamber that expand s and contract s, depending on the atmospheric pressure around it.

Mechanical tools measure how much the chamber expands or contracts. These measurements are align ed with atmospheres or bars. The aneroid barometer has a circular display that indicate s the present number of atmospheres, much like a clock. One hand moves clockwise or counterclockwise to point to the current number of atmospheres. The terms stormy, rain, change, fair, and dry are often written above the numbers on the dial face to make it easier for people to interpret the weather.

Aneroid barometers slowly replaced mercury barometers because they were easier to use, cheaper to buy, and easier to transport since they had no liquid that could spill. Some aneroid barometers use a mechanical tool to track the changes in atmospheric pressure over a period of time. These aneroid barometers are called barographs. It is a simple, seemingly obvious notion: air has weight; the atmosphere presses down on us with a real force.

The same was true for early scientists, who never thought to consider the weight of air and atmosphere. This same revelation also led Torricelli to discover the concept of a vacuum and to invent the barometer — the most basic, fundamental instrument of weather study.

On a clear October day in , Galileo conducted a suction-pump experiment at a public well just off the market plaza in Florence, Italy. They pumped until the tube flattened like a run-over drinking straw. But no matter how hard they worked, water would not rise more than 9.

It was the same in every test. Galileo proposed that—somehow—the weight of the water column made it collapse back to that height. In , Torricelli returned to the suction pump mystery. If Galileo was correct, a heavier liquid should reach the same critical weight and collapse at a lower height.

Liquid mercury weighted Torricelli filled a six-foot glass tube with liquid mercury and shoved a cork into the open end. As he expected, mercury flowed out of the tube and into the tub. But not all of the mercury ran out. The next day, with wind and a cold rain lashing at the windows, Torricelli repeated his experiment, planning to study the vacuum above the mercury.

However, on this day the mercury column only rose to a height of 29 inches. Torricelli was perplexed. He had expected the mercury to rise to the same height as yesterday. What was different? Rain beat on the windows as Torricelli pondered this new wrinkle.

What was different was the atmosphere, the weather. Air, itself, had weight. Other scientists soon begin trying to determine the properties of this "air. In he publishes his book on gases. Over the next decade they work, independently of each other, to identify many new "airs.

It would be many more years before oxygen's role in respiration would be clearly understood. While some such as Priestley stand by the phlogiston theory, by the late s, most have converted.