The Magdeburg hemispheres are a pair of large copper hemispheres with mating rims. When the rims were sealed with grease and the air was pumped out through the valve below the lower hemisphere, the lowered pressure within the sphere made it very difficult to pull the spheres apart: a famous image shows teams of horses pulling in opposite directions unable to part the spheres. This device was designed by German scientist Otto von Guericke in 1650. von Guericke was also the mayor of Magdeburg, thus the device name. This device is still used as a teaching tool to demonstrate the force of air pressure. This version was made by Pike in New York.