Magdeburg Hemispheres are two hollow hemispheres made of some strong material, such as brass or gun-metal, which can be accurately fitted on each other. Before putting them together it is usual to smear the edges with grease. The hollow sphere then formed is exhausted of air through a stopcock with which one of the hemispheres is provided. After the exhaustion is complete it is found that an immense force is needed to pull the two parts asunder, the force increasing with the diameter of the hemispheres. The two hemispheres are pushed together by the pressure of the atmosphere; so, if a complete vacuum were obtained the force required to separate the parts would be 15 lbs. multiplied by the number of square inches in the base of either hemisphere.