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"We live at the bottom of an ocean of the air element, which, through an unquestionable experience, is shown to have weight." These words were pronounced by the Italian physicist and mathematician evangelist Torricelli, after the manufacture of the first barometer in the 17th century.

Evangelist Torricelli experimented with layered glass tubes and various liquids of different densities until he used Mercury, with which he managed to balance atmospheric pressure. Torricelli's experiment reached a great popularity, but it was the subject of fierce controversy.

Blaise Pascal studied the experiment of Torricelli and concluded that the barometric space was empty, which confirmed the atmospheric pressure as a cause of the balance of the mercury column and his study contributed to the establishment, on the part of Pascal, from the beginning of Pascal.

According to Pascal's principle, the pressure exerted on a liquid is transmitted equally in all directions. At the initiative of Pascal the barometer was started to be used in meteorological observations. The English physicist Robert Boyle, baptized the barometer and with him discovered the law on the gases that bears his name.

During the eighteenth century the accuracy of the barometers was increased by adding more precise scales with verniers and the consideration of the errors that caused the capillarity and temperature. At the end of that century they were already used for the measure of heights.

I prefer use this app: https://play.google.com/store/apps/details?id=ah.creativecodeapps.tiempo&hl=en but you have here more types of barometer

Types of Barometer
Although there is a wide variety of tools used by meteorologists to measure and predict climate cycles, for example weather vanes measure wind direction and strength and thermometers indicate ambient air temperature, barometers (which They measure atmospheric pressure) are one of the most important instruments in the weather forecast because they are very clear the types of climate that bring with them high and low pressure systems.

A barometer is used to measure air pressure, with atmospheric pressure being the weight per unit of surface exercised by the atmosphere. Changes in air pressure can precede a big storm. Barometers tend to fall if the rainy weather is imminent and rising if the climate will soon clear, which means that high pressures correspond to regions without precipitation, while low pressures are indicators of storm regions.

The first barometers were formed by a column of liquid enclosed in a tube whose upper part is closed. The weight of the fluid column compensates exactly for the weight of the atmosphere. The unit of measurement of the atmospheric pressure that usually mark the barometers is called hectopascal, of abbreviation HPa. This unit means: Hecto which is equal to a hundred and pascals which is the unit of measurement of pressure.

The mercury barometer, the most common and used of the barometers, often determines the measuring unit, which is referred to as "inches of mercury" or "millimeters of Mercury" (abbreviated MmHg method). A pressure of 1 mmHg is 1 Torr (per Torricelli).

The mercury barometer was invented by Torricelli in 1643. A mercury barometer consists of a glass tube about 850 mm high, closed by the upper end and opened by the lower. The tube is filled with mercury, inverted and the open end placed in a container full of the same liquid. If it is uncovered, it will be seen that the tube mercury descends a few centimeters, leaving at the top an empty space (barometric chamber or Torricelli vacuum). Thus, the mercury barometer indicates the atmospheric pressure directly by the height of the Mercury column.

Barometer 01

Aneroid Barometer is a barometer that does not use mercury. It indicates the variations of atmospheric pressure by the more or less large deformations that the one makes experiment to a metallic box of very elastic walls in whose interior the most absolute emptiness has been made. It is graduated by comparison with a Mercury barometer, but its indications are more and more inaccurate because of the variation of elasticity of the plastic spring. It was invented by Lucien Vidie in 1843.

Barometric altimeters are used in aviation and are essentially barometers with the scale converted to meters or feet of altitude, while the Fortin barometer is composed of a Torricelliano tube that is introduced into the mercury contained in a bucket of Glass in tubular form, provided with a base of Gamo leather whose shape can be modified by means of a screw that rests on the tip of a small ivory cone. That's how you keep a fixed level.

This barometer is completely covered with brass, except for two vertical slots next to the tube that allow to see the mercury level. In the front slot there is a graduation in millimeters and a vernier for the reading of tenths of millimeters. In the posterior one there is a small mirror to facilitate the visibility of the level. A thermometer is attached to the barometer.

Fortin barometers are used in scientific laboratories for high precision measurements and readings should be corrected taking into account all factors that may influence them, such as ambient temperature, gravity acceleration of place and the vapor tension of Mercury, among others.

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