To demonstrate Atmospheric Pressure. Consists of two metal Hemispheres 4""(10cm) in diameter, each with a sturdy metal handle. Join the two Hemispheres and connect to a hand pump or vacuum pump and evacuate the air in the Hemispheres. One of the spheres has a stopcock to attach vacuum tubing 8mm ID. Try pulling the Hemispheres apart, you will find it very hard, if not impossible to pull them apart. Upon opening the stopcock and letting the air in the Hemisheres can be easily pulled apart.