
Swiss physicist and mathematician Daniel Bernoulli's 1738 pressure apparatus diagram.

Swiss physicist and mathematician Daniel Bernoulli's 1738 pressure apparatus diagram.

In thermodynamics, pressure is a quantity of tension whose conjugated extensity is volume, defined as force per unit area of surface; formulaically:

The unit of pressure is the Pascal, named after Blaise Pascal. [1] A device for measuring atmospheric pressure is called the barometer.

History
In 1643, Evangelista Torricelli, during his investigation into the pump problem, demonstrated the so-called "Torricellian vacuum", so to explain why hand operated water draining pumps stopped working past a depth of about 32 feet. The Torricellian vacuum found that air pressure is equal to the weight of 30 inches of mercury. If air has a finite weight, earth's atmosphere must have a maximum height.

In 1648, Pascal reasoned that if it true, according to the results of the Torricelli vacuum (1643), were that earth's atmosphere must have a maximum height, air pressure on a high mountain must be less than at a lower altitude. He resided near the Puy de Dome mountain, 4,790 feet (1,460 m) tall, but his health was poor so could not climb it.

On 19 Sep 1648, after many months, Pascal convinced Florin Perier, husband of Pascal's elder sister Gilberte, to make the climb; Perier described the results as follows: (Ѻ)

“At eight o'clock, we met in the gardens of the Minim Fathers, which has the lowest elevation in town. First, I poured 16 pounds of quicksilver into a vessel, then took several glass tubes, each four feet long and hermetically sealed at one end and opened at the other. Then placed them in the vessel of quicksilver. I found the quicksilver stood at 26" and 31⁄2 lines above the quicksilver in the vessel. I repeated the experiment two more times while standing in the same spot. They produced the same result each time. Taking the other tube and a portion of the quicksilver, I walked to the top of Puy-de-Dôme, about 500 fathoms higher than the monastery, where upon experiment, and found that the quicksilver reached a height of only 23" and 2 lines. I repeated the experiment five times with care. Each at different points on the summit, found the same height of quicksilver, in each case.”

In 1738, Daniel Bernoulli, in his Hydrodynamica, described pressure of a gas as being due to the bombardment of the particles of the wall of the containing vessel; he defined pressure as follows: [3]

“The weight P holding down the piston in [a given] position is the same as the weight of the overlying atmosphere, which we shall designate P in what follows.”

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Human systems
The translation and understanding of the standard definition of pressure in systems of human molecules is a very difficult subject, as is the case with heat, temperature, and volume. Standard atmospheric barometers, for instance, are not designed or capable of measuring subtle human system pressures, such as to account for phenomena such as variations in territory densities or high school cafeteria seating distributions of alpha-, beta-, and gamma- males and females. [2]

Quotes
The following are related quotes:

“The weight of air on the earth’s surface is as great as the weight of water about 20 Magdeburg cubits deep. In other words, if water should rise 20 cubits above the earth’s surface, the pressure it would exert on all things beneath is the same as the pressure of air.”
— Otto Guericke (1663), Magdeburg Experiments on the Vacuum of Space (pg. 113)

See also
● Economic pressure
● Social Pressure

References
1. Perrot, Pierre. (1998). A to Z of Thermodynamics, Oxford: Oxford University Press.
2. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (preview), (ch. 9: "Human Molecular Orbitals", section: Orbtial applications: pgs. 283-295) Morrisville, NC: LuLu.
(b) Thims, Libb. (2006). "Study: High School Cafeteria Seating Distributions", Chicago: IoHT publications.
3. (a) Bernoulli, Daniel. (1738). “On the Properties and Motions of Elastic Fluids, Especially Air” (Hydrodynamica, Section 10) in: The Kinetic Theory of Gases of Gases (pgs. 57-65), 2003, by Stephen G. Brush, Nancy S. Hall. Imperial College Press.
(b) Bernoulli, Daniel. (1738). Hydrodynamica, Sive Vivibus et Motimus Fluidorum Commentarii. Sectio Decima: “De affectionibus atque botimus fluidorum elasticorum, praecipue autem aeris.” (pgs. 200-204). Argentorati, Sumptibus Johannes Reinholdi Dulseckeri.

External links
● Pressure – Wikipedia.