A bucketful of air weighs only a little. But our ocean of air is so deep that it pushes on everything on the earth with a great deal of force. Invented in the mid-17th century, barometers measure this force.
In some places the air pushes harder than in other places. At the top of a mountain, for instance, it does not push so hard as in the valleys near by. There is not so much air above to press down. The air pressure, we say, is less on the mountaintop. Even in the same place, the air pressure does not stay the same day after day. It changes with the weather.
Because barometers can measure changes in air pressure, they are very useful to the weatherman. They help him predict the weather. They are very useful to airplane pilots, too. They help tell how high a plane is above sea level. They help mountain climbers in the same way.
There are different kinds of barometers. In some barometers there is a tube which has mercury in it. Mercury barometers look a little like big thermometers.
In other barometers there are one or more little boxes with a partial vacuum inside. Barometers of this kind are called aneroid barometers. An aneroid barometer has a hand that moves whenever the air pressure changes.
Some barometers keep a record of the air pressure. A pen on the hand of the barometer makes a mark on a moving sheet of paper.
