The two hemispheres, having the same diameter and the edges perfectly smooth, fit each other perfectly and when placed one on top of the other they form an airtight sphere. A leather gasket with a uniform coat of grease assured a good seal. The lower hemisphere has a cock and can be unscrewed from the staff holding it to be screwed to the plate of a pneumatic machine; the upper one has a ring used to apply traction on it once the vacuum has been created and the cock closed.
When the two hemispheres contain air they can be separated easily; once a vacuum has been created and the cock has been closed great strength is required to pull them apart: the larger the hemispheres the greater the force needed to separate them. Even when the vacuum is not total the force applied by each hemisphere on the other is practically equal to that of atmospheric pressure (equal to 1.03 kg per square centimetre of the surface delimited by the circular edge of the two hemispheres). In one of the spectacular public demonstrations performed by von Guericke at Regensburg (Ratisbon) in 1654, in the presence of Ferdinand III, Emperor of Germany, he used two hemispheres 44 cm in diameter. Once the vacuum had been created, only a great effort by eight pairs of horses succeeded in separating them, with a strong detonation.