Man's physical universe



Pressure Is Generally Measured with a Mercurial Barometer.

Evangelista TorriceUi (1608-1647) in 1643, the year after the death

of his teacher, Galileo, filled a glass tube, closed at one end, with mercury,

immersed it in a bath of mercury, and had the pleasure of seeing

the mercury lev^el drop to a height of 30 inches, which his previous calculations

had led him to expect it to do. The vacuum left above the mercury

in a barometer has ever since been called the Torricellian vacuum.

Torricelli's experiments were received with incredulity because they

were contrary to the long-accepted teaching of the Greek philosophers

Fig. 65. A simple mercurial


that "nature abhors a vacuum." TorriceUi

died before he could convince people that nature

abhors a vacuum only up to the extent of the

pressure of the atmosphere. In 1648 Blaise

Pascal (1623-1662) repeated Torricelli's experiments,

using a 50-foot tube and water for


liquid. He found that the water rose in the

tube to a height of about 34 feet, which corresponds

to 30 inches of mercury, water being

about 1/13 the density of mercury.

In 1647 Rene Descartes wrote to Pascal suggesting

that the height to which the mercury

would rise in the mercurial barometer would be

less at higher altitudes where the atmospheric

pressure is less. Pascal had his brother-in-law.

Florin Perier, conduct this experiment for him.

Perier took a tube to a mountain top and found

that the mercury level was 3 inches lower than

at the foot of the mountain. On the basis of

this experiment Pascal worked out the use of the barometer for measuring

heights above sea level.

Various accessories are added to modern barometers to permit precision

in reading the height of the mercury in the tube above the level

of the mercury in the cistern, in order to measure the small rise or fall of

mercury in the tube in response to slight increases or decreases of the

atmospheric pressure. A thermometer is mounted on the barometer

because corrections must be made for temperature.

The Aneroid Barometer Is Portable.

The sensitive part of the aneroid barometer consists of a small,

airtight, corrugated metal box with the air exhausted and a flexible

cover held in position by a spring. Changes in air pressure move this

cover in or out, which motion is communicated to a movable pointer

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