Magdeburg hemispheres, 1761.
© Science Museum / Science & Society Picture Library
In the mid 17th century Otto von Guericke used an air pump to remove the air from between two large close-fitting hemispheres. Teams of horses could not separate them. A scaled down version carried out with air pumps became a standard demonstration. If the air was removed from between the hemispheres, a weight of 40 lb was required to separate them. However, if they were placed in a bell jar and the air was removed, they fell apart. These hemispheres were made by George Adams, instrument maker to the king.