|Diagram of the Magdeburg hemispheres showing an internal vacuum about which the massive weight of the surrounding atoms and molecules of the atmosphere press inward on the spheres.|
In this vacuumed-out state, the pressure of the weight of surrounding atmosphere, piled upwards of 62-miles above the sphere, as is the case for all bodies on the surface of the earth, to the Karman line earth-space boundary where point weightlessness begins, acts to hold the spheres together tightly with great force by pressing inward on the outer casing.
The Magdeburg hemispheres were invented by German engineer Otto Guericke who some time near the end of the Thirty Years’ War (1618–1648), became mayor of Magdeburg (hence the name), from 1646 to 1676, and in this period began to devote a considerable portion of his spare time to experimentation and was especially fascinated with the nature of cold, becoming very curious on the the question: 
“Could empty space exist, and is heavenly space unbounded?”
In researching this query, he was brought into contact with German mathematical physicist Gaspar Schott, an adherent to Aristotle’s version of the denial of the void, albeit open to new experimental information, and ultimately and to French scientist philosopher Rene Descartes’ adherence to the “denial of the vacuum” dictum.
|Left: a Schott diagram depiction of German engineer Otto Guericke's circa 1648 attempt to make a vacuum inside of tightly-sealed beer keg, which introduced him to the so-called "sealing problem", i.e. how to seal in the vacuum.  The two men seem to be pulling on the the world's first piston and cylinder prototype, something Guericke would later make, a precipitate of his vacuum research. Right: a Schott diagram depiction of Guericke's solution to the sealing problem: two fitted copper spheres sealed with grease around the rim, spheres which would latter become famously known as the Magdeburg hemispheres. |
This vacuum puzzle intrigued Guericke and he went to work trying to evacuate the air form a well-caulked beer keg, which introduced him to the sealing problem, i.e. how to make a container air tight.
After solving the sealing problem, he was said to have discovered the phenomenon of the compressibility of air after which he invented a vacuum pump. Using the vacuum pump as a tool, the Magdeburg hemispheres were eventually built some time around 1853-1857.
There seems to be a bit of a discrepancy as to when and where the actual first public demonstration of Magdeburg hemispheres occurred. Supposedly, legend has it that the first show appeared in 1654 in Regensburg. This, however, may be only legend, and it is said that the actual first appearance of the horse pulling show occurred in Magdeburg in 1657 and was later repeated in Berlin in 1663.
Gaspar Schott’s 1657 drawing showing that it required 16 total horses to separate two 22” hemispheres is said to be the first public experiment, or at least the most popular or best known version.
Other references state that in 1854 to the Reichstag and the Emperor Ferdinand III in Regensburg, where it was found that 30 horses, in two teams of 15, could not separate the hemispheres until the vacuum was released.
Whatever the case, the public demonstrations showed that vacuums do exist and thus publically disproved Greek philospher Parmenides 485BC nature abhors a vacuum postulate, which supposedly was the point of building the hemispheres in the first place.
Guericke is said to have calculated that a force of 2,700 pounds would be needed to pull the hemispheres apart, as is depicted in his 1672 lifting experiment.  This would have been the one of the first measurement of the "force of the vacuum" or "power of the vacuum" an elusive entity that would searched for as a potential source of man power in the follow designs of the vacuum engine in the decades to follow.
In 1657, German physical mathematics professor Gaspar Schott described the experiment in his Mechanica Hydraulico-Pneumatica.  Upon first seeing the demonstration of the hemispheres Schott is said to have remarked: 
“I have no reservations to say frankly and courageously that I have never seen, heard, read or comprehended anything more wonderful than this.”
In 1663, Guericke repeated the demonstration with 24 horses in Berlin before Frederick William, Elector of Brandenburg. The experiment was said to have been displaced all over Europe in these years.
The original Magdeburg hemispheres (left) and Guericke's vacuum pump (right), in Deutsches Museum, Munich, Germany, at the top of which is shown vacuum bulb, which could be disconnected and carried about as a sort of power or storage vessel a sort of proto type battery.
Close-up view of the hemispheres (left) showing the connectors and stopper valve, which would release the vacuum when desired.
An advertisement (below) depicting entitled "Curious Experiences in Physics: The Magdeburg Hemispheres".
As is the case with most advances in science, Guericke's vacuum machine was not without implication. It is said that Guericke intuitively was aware of the delicacy of his experiments and therefore avoided theological discussions by using a clever linguistic ploy. He replaced the critical word “emptiness” with a simple physical one by distinguishing between “emptiness” and “empty space”. His empty space did not mean “void” but it was just an air-evacuated space. Using this ploy he was able to carry out his spectacular and meanwhile world-famous experiments with the Madgedurger Hemisphere without being unchallenged. 
|Depiction of a 24-horse version of the Magdeburg hemispheres from the 1899 book Album of Science: Famous Scientist Discoveries. |
An 8-horse 1662 Magdeburg hemispheres experiment.
A 1994 Magdeburg hemispheres re-enactment.
Parts to the hemispheres and the vacuum pump.
A Magdeburg hemisphere statue
|Magdeburg hemispheres shown partially evacuated (left) and in balance with the atmosphere with no vacuum (right). |
1. Otto von Guericke and the Magdeburg Hemispheres – MK-Technology.com.
2. Magdeburg hemispheres (24 horses) – from the 1899 book Album of Science: Famous Scientist Discoveries.
3. Gaspar Schott – Wikipedia.
4. Dunlavy, Mike (2008). “Magdeburg hemispheres”, Nov. 07, SMU Astronomy and Physics.
5. Magdeburg hemispheres – Physics.Kenyon.edu.
6. Partners and Rivals during the Scientific Revolution – Faculty.Fairfield.edu.
7. Guericke, Otto and Schott, Kaspar. (1672). Otto Guericke’s New Experiments: on (as they are called) on the Magdeburg vacuum space (Ottonis De Guericke Experimenta Nova (ut vocantur) Magdeburgica de Vacuo Spatio) (15+ diagrams, various pages) (preface, pdf). Janssonius a Waesberge.
● Magdeburg hemispheres – Wikipedia.