The instrument used to measure air pressure is called a barometer. Barometers come in two basic forms: the mercury barometer and the aneroid barometer. A mercury barometer has a long glass tube filled with mercury turned upside down in a bowl of mercury known as the cistern. As the mercury runs out of the tube and goes into the cistern, it creates a vacuum at the top of the tube. Air pressure is responsible for keeping the column of mercury up, and as air pressure pushes down on the mercury in the cistern, it forces more mercury up the tube. An aneroid barometer uses cells that expand and contract as air pressure changes. Air pressure is measured by attaching a needle into these cells.