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J. Perelman
"Entertaining physics". Book 2.
Chapter 6. Properties of liquids and gases

THE LOAD FROM THE AIR

In the mid-seventeenth century, the inhabitants of the city of Regensburg and came back the sovereign princes of Germany, headed by the Emperor witnessed the amazing spectacle: 16 horses struggled to separate the two attached to each other copper hemisphere. That linked them? "Nothing" - the air. Nevertheless, eight horses pulling to one side, and eight, pulled in another, was unable to disconnect. So the mayor Otto von Guericke clearly demonstrated to all that the air is not "nothing"that it has weight and exerts considerable force on all earthly things.

This experience was made on 8 may 1654 at a very festive atmosphere. Scientist mayor managed all be interested in their research, despite the fact that it happened in the midst of the political turmoil and devastating wars.

Description famous experience with "Magdeburg hemispheres" is available in the physics textbooks. Still, I'm sure the reader interested to hear this story from the mouth of the Gerike, sawyers, "German Galileo", as it is sometimes called wonderful physics. Voluminous book with a description of a long series of his experiences was published in Latin in Amsterdam in 1672, and, like all books of this era, wore a lengthy title. Here it is:

OTTO von GUERICKE

The so-called new Magdeburg experiments
over the AIRLESS SPACE,
originally described by Professor of mathematics
in Wuerzburg University CASPAR SCHOTT.

Publication of the author,
more detailed and updated with various
new experiments.

We are interested to experience dedicated Chapter XXIII of this book. Here is a literal translation.

"Experience proves that the air pressure connects the two hemispheres so firmly that they cannot be separated by the efforts of 16 horses.

I ordered two copper hemispheres with a diameter of three quarters of Magdeburg elbow*. But in reality, their diameter is made just 67/100, as master, as usual, could not produce exactly what was needed. Both hemispheres met one another. One hemisphere was fixed crane; with this valve you can remove the air from the inside and to prevent the infiltration of outside air. In addition, the hemispheres were attached 4 rings, through which is passed through the ropes attached to the harness of horses. I was told to sew leather ring; it was nourished by a mixture of wax in turpentine; trapped between the hemispheres, it did not pass air into them. The crane was inserted tube of the air pump, and was removed the air inside the balloon. Then it was discovered that the strength with which the two hemispheres was ridge press down to each other through the leather ring. The pressure of the outside air was pressed them so hard that the 16 horses (jerk) was not able to pull them apart or reached only with difficulty. When the same hemisphere, yielding the voltage across the power of horses, coming apart, I heard a roar, as from a shot.

But it was necessary to turn the valve to open free access to the air - and hemisphere was easy to separate hands."

A simple calculation can explain to us why such a significant power (8 horses on each side)to disconnect parts of the empty bowl. The air presses with a force of about 1 kg per sq. cm; the area of a circle**diameter 0.67 elbow (37 cm) is equal to 1060 cm -2. Hence, the pressure of the atmosphere on each hemisphere must exceed 1000 kg (1 tonne). Each eight horses had, therefore, to pull with a force of tons, to counteract the pressure of the ambient air.

It would seem, for eight horses (each side) this is not a very big load. Do not forget, however, that by moving, for example, carry 1 ton, horses overcome the force is not a 1 ton, and a much smaller, it is the friction of the wheels about the axis and on the pavement. And this power is on the highway, for example, only five percent, i.e., when the solid load - 50 kg (Not talking about the fact that when the combined efforts of eight horses lost, as practice shows, 50% thrust.) Therefore, thrust 1-ton corresponds with eight horses load carts 20 tons. That's what the air load, to drive the horses burgomaster of Magdeburg! It seemed like they had to move a small steam locomotive, not supplied, moreover, on the rails.

Measured that a strong draft horse pulls a cart with a force of only 80 kg***. Therefore, to break the Magdeburg hemispheres would be under uniform traction 1000/80 = 13 horses on each side.

The reader would probably be astonished to learn that some of articulation of our skeleton do not fall for the same reason as the Magdeburg hemispheres. Our hip joint is a such Magdeburg hemispheres. You can expose this joint from muscular and cartilaginous connections, and still hip does not fall: it presses the atmospheric pressure, as in the interarticular space there is no air.


The bones of our hip joints do not break due to the atmospheric pressure,
just as restrained by the Magdeburg hemispheres.

* "Magdeburg elbow" is equal to 550 mm
** Does the area of a circle, and not the surface of the hemisphere, because the atmospheric pressure is equal to the specified value only if the action on the surface at a right angle; sloping surfaces this pressure is less. In this case we take the rectangular projection of the spherical surface to a plane, i.e., the area of the big circle.
*** At a speed of 4 km per hour. On average, it is assumed that the thrust force of the horse is 15% of its weight; it weighs the same horse: an easy - to 400 kg, heavy - 750 kg For a very short time (initial stress), the thrust force can be several times more.

Entertaining physics J. Perelman

 




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