Apparatus consisting of two hollow metal hemispheres fastened together. When a partial vacuum is produced inside them, considerable force needs to be applied in order to overcome atmospheric pressure and separate the hemispheres. In c. 1655, Otto von Guericke (1602-1686), chief magistrate of Magdeburg, developed an air pump that could extract air from sealed vessels. The new instrument enabled him to set up a spectacular experiment witnessed by vast crowds of local residents in 1657. Von Guericke demonstrated that the weight of air pressed together two perfectly fitting hemispheres (inside of which a vacuum had been created by means of the air pump) with such force that two teams of eight horses pulling in opposite directions were needed to separate them. Von Guericke sensed that the weight of air was a force that could be harnessed for work such as lifting weights. He thus initiated a line of research that led to James Watt's (1736-1819) steam engine.