In 1643 the Italian mathematician and physic Evangelista Torricelli (Galileo's pupil) experimented with a pipe fulfilled by mercury. He smelted the pipe 1 meter long on one end and fulfilled by mercury the whole pipe. He packed the other end by thumb, than he turned it up by the bottom and he put the end closed by the thumb to the dish with mercury. When he realised his thumb the level fell down however but it was always higher than the level in the dish. In the higher part of the pipe there was created vacuum long about 250 mm. It was the first known vacuum that was created in artificial way. Torricelli judged that mercury in the pipe was kept by the atomic mass of air which was expressed itself by a press on mercury in the dish. So he proved the existence of atmospheric press and the material nature of air.

The French mathematician  Blaise Pascal repeated Torricelli's experiment but he used a red wine instead of mercury (!). Because wine is 15 times lighter than mercury the column of wine was so 15 times higher than the column.of mercury. Blaise Pascal with his brother climbed up on a nearby hill of Puy de Dome (1054 m) with a similar equipment to test Torricelli's hypothesis and he learned that the higher they climbed up the more the level of mercury fell down (about 76 mm). He understood that it was because of the press of air was falling down with increasing an elevation above the see level. This way the instrument to measuring of press was invented. Robert Boyle is credited with naming the Torricelli's  tube a barometer. Barometer comes from the Greek word baros (weight) and metron (to measure).

The puzzle was to explain why there were daily fluctuations in air pressure.

In 1672 Otto von Guericke noticed that the high press meant a good weather and contrary the low press meant to be worse. He constructed the similar instrument like Torricelli but in a greater standard to prove his examination. He provided the upper end of brass pipe long 10.4 meters by a extension piece of glass and than he fulfilled it with water and drowned it into the barrel with water. There was swimming the small figure of man in the extension piece. He put the instrument on the out- side wall of a home. During a good weather the small man swam high and during a bad weather he swam low. He was not amusing his neighbours the thought that he performed magic.

The Fortin's barometer was invented in 1810 and really all it did was adjust the height of the column of mercury.

The mercury barometer. The container at the bottom, the scale at the top.

The next step in barometers was to make them easy to move and easy to construct. So, in 1843 Lucien Vidie, French scientist, invented the aneroid barometer. The aneroid barometer (without liquid) is based on a similar principle but there is a flexible box without air and its upper surface is connected with a lever. Changes in atmospheric pressure caused the box to expand or contract. They were then connected to lever and this lever carries up the moving to a hand moving on a marked scale.

Classical combination of the aneroid barometer with the thermometer.

The barograph is an aneroid barometer that has a pen attached to its indicator and the pen traces out the barometric readings on a rotating drum giving a continuous trace of air pressure.

Barograph. The drum is on the left, in the middle flexible box
(all images taken over from  http://www.9online.com/wx/whys/baro.htm)

The hectopascal is the standard unit of atmospheric pressure, sometimes the millibar is used (1 hPa = 100 Pa = 1 mb).

The standard atmosphere is the pressure exterted at sea level excerting a pressure of 1013,2 hPa.