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### Measurement of Air Pressure

We often read that the pressure of the atmosphere is 30 in. on 76 chi. What does this mean? Let us consider the construction of a simple barometer.

Experiment.

Take a thick-walled glass tube about one yard long, closed at one end, and fill it with mercury until the liquid is about one inch from the open end. Place a finger firmly over this end and invert the tube several times: this removes the small bubbles of air clinging to the sides of the tube. Now completely fill the tube with mercury, again place a finger over the open end and invert the tube placing the open end under the surface of mercury in a dish. Remove the finger and immediately the mercury in the tube falls a short distance. Support the tube with a stand and clamp.

The space above the mercury is known as a Torricellian vacuum after Torricelli, an Italian assistant of Galileo, who constructed the first barometer in 1643. The mercury column in the tube is supported by the pressure of the atmosphere on the surface of the mercury in the dish. Thus the height of the mercury column may be used as a measure of the atmospheric pressure. Since water vapour is less dense than dry air, the presence of water vapour in the atmosphere reduces the pressure and the height of the mercury column is reduced, that is the barometer falls. If the atmosphere becomes drier and more dense, the height of the mercury column increases and the barometer rises. Thus a barometer may be used to measure the value of atmospheric pressure. When we say that the barometer reading is 30 in., we mean that the pressure of the atmosphere is such that it can support a column of mercury 30 in. high. The barometer is also used in a variety of weather glasses which indicate the changes to be expected in the weather. The barometer is one of the key instruments used by the meteorologist in preparing his weather forecasts. An aneroid barometer is a further type which contains no liquid. In this the readings are given by the effect of atmospheric pressure on a metal box partially exhausted of air.

as being made up of зд. как состоящего из

by pressure зд. под давлением

compared with по сравнению

Galileo Галилей

Inch дюйм (2,5 см)

thick-walled толстостенный

Torricelli Торичелли

water vapour водяной пар

when contained зд. когда они находятся

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