Suction pumps were already known to ancient civilizations. Water could be raised to a considerable height with their aid. Water very obediently followed the piston of such a pump.

Ancient philosophers thought about the causes for this and arrived at the following profound conclusion: water follows the piston because nature fears a vacuum and so does not leave any free space between the piston and the water.

It is told that an artisan constructed for the Duke of Tuscany in Florence a suction pump whose piston was supposed to draw water to a height of more than 10 m. But no matter how they tried to begin sucking up water with this pump, nothing came of it. The water rose 10 m with the piston, but after that the piston left the water behind, and so the very same vacuum which nature fears was formed.

When Galileo was asked to explain the cause of this failure, he answered that nature really dislikes a vacuum, but only up to a certain point. A disciple of Galileo, Evangelista Torricelli (1608–1647), evidently used this case as an excuse to perform his famous experiment in 1643 with a tube filled with mercury. We have just described this experiment—the constructing of a mercury barometer is precisely Torricelli’s experiment.

Taking a tube of height more than 76 cm, Torricelli created a vacuum over the mercury (it is often called a Torricellian vacuum in his honour) and thus proved the existence of atmospheric pressure.

By means of this experiment, Torricelli cleared up the misunderstanding of the Duke of Tuscany’s artisan. In fact, it is easy to see how many metres water will humbly follow the piston of a suction pump. This motion will continue until the column of water with an area of 1 cm\(^2\) acquires a weight of 1 kgf. Such a column of water will have a height of 10 m. This is why nature fears a vacuum …, but only up to 10 m.

In 1654, 11 years after Torricelli’s discovery, the action of atmospheric pressure was graphically demonstrated by the Burgomaster of Magdeburg, Otto von Guericke (1602–1686). It wasn’t so much the physical essence of the experiment as the theatricality of its performance that brought the author renown.

Two copper hemispheres were connected by an annular washer. The air was pumped out of the sphere so obtained through a pipe attached to one of the hemispheres, after which it was impossible to separate the hemispheres. A detailed description of Guericke’s experiment has been preserved. The atmospheric pressure on the hemispheres can now be calculated: for a diameter of 37 cm, the force was approximately equal to 1000 kgf. In order to separate the hemispheres, Guericke ordered that two teams of eight horses each be harnessed. Ropes passing through the rings attached to the hemispheres were tied to the harnesses. The horses proved unable to separate the Magdeburg hemispheres.

The forces supplied by eight horses (exactly eight and not sixteen, since the second team harnessed for greater effect could have been replaced by a hook nailed to the wall, with no change in the force acting on the hemispheres) were not enough to break the Magdeburg hemispheres.

If there is a cavity between two bodies in contact, these bodies will not come apart because of atmospheric pressure.