Caption: Otto von Guericke's (1602--1686) famous demonstration that teams of eight horses could NOT pull apart two copper hemispheres of diameter 51 cm which had been evacuated and were being held together only by ambient air pressure (see Wikipedia: Otto von Guericke: Air pressure and the vacuum).

Note this is a demonstration experiment, NOT a controlled experiment intended to rigorously prove something.

The force of air pressure sealing the hemispheres is about

  p*A = p*π*r**2 
      ≅ (1.01*10**5 Pa)*π
         *(0.255**2) 
      ≅ 20632 N ≅ 4638 lb.  

Considering that in modern horse pulling (which is NOT the sport of kings), a pair of draft horses can pull with up to about 5000 lb of force for distances of some tens of feet, von Guericke's horses must have been feeble beasts---ponies.

See Horse Pulling Records for horse pulling records and images.

By the by, Aristotle (384--322 BCE) in his Aristotelian physics determined that a vacuum could NOT exist based on a priori reasoning and some empirical evidence (see Wikipedia: Aristotelian physics: Continuum and vacuum; Fu-188--193). In fact, Aristotle reportedly said "nature abhors a vacuum" (in Latin "Natura abhorret vacuum") (see Wikipedia: Horror vacui: History) and really did say "the so-called vacuum will be found to be really vacuous" (Fu-190). Of course, Aristotle was wrong: the vacuum exists if never perfectly. But Aristotle was also right since the modern understanding of vacuum is that it is NOT an inert nothingness, but an active quantum vacuum state in which there is always at least a QED vacuum (the lowest energy state of the electromagnetic field) and where virtual particles are always appearing and disappearing.

Credit/Permission: Gaspar Schott (1608--1666), 1657 (uploaded to Wikimedia Commons by User:Torsten Schleese, 2006) / Public domain.
Image link: Wikimedia Commons: File:Magdeburger-Halbkugeln.jpg.
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