The Magdeburg Hemispheres
The Magdeburg hemispheres consist of hollow hemispheres, one with a valve to let out air. The hemispheres are placed together with a seal provided by a greased leather ring between the hemispheres. The air between the hemispheres is pumped out and the valve is closed.
Before the air is pumped out the pressure inside and outside of the spheres is the same, so there is no net force holding the spheres together.
After the air is pumped out there is a pressure difference
between the inside and the outsides. If the total area of the hemispheres is
the force holding them together is
The hemispheres were first demonstrated before an Imperial court in Germany, when two teams of eight horses proved unable to separate the spheres.until air was allowed through the valve.