BAROMETER

The air around you is heavy. It has weight. There is a ton of air over each square foot of the earth’s surface. There is a name for the air’s weght pressing down on the surface of the earth. The name is air pressure. A barometer is an instrument that can measure this air pressure.
You may have seen a barometer hanging on a wall. In it there is a small box, or chamber, with thin metal walls. Most of the air has been drawn out of this chamber. The thin walls move it or out slightly when there is an increase or decrease in the air pressure outside. A pointer on the front of the barometer is connected by a lever to the chamber walls and shows these small changes. The pointer moves in front of a dial that is marked off in units of air pressure. The units are usually inches of centimeters.
Air pressure is important in wheater forecasting. A change in air pressure usually means change in weather. A drop in pressure often means that bad weather is coming. A rise in pressure ussualy means that clear weather is on the way. Ship and airplanes carry barometers. Weathermen use barometers in making their forecasts.
Air pressure alse changes when you move from sea level to higher altitudes. When you climb a mountain, you find that the air grows thinner. It weighs less, and so it presses down less strongly. Air plane pilots often use barometers called altimeters to tell them how high they are above the ground.
The barometers you have read about so far are called aneroid barometers. Aneroid means "without liquid"." There are other barometers that do use liquids. The very first barometers was of this type. An Italian scientist named Evangelista Torricelli (1608-47) designed it closed at one end and filled it with mercury. He turned this tube upside down in a pan that contained more mercury. The mercury in tube started running out, of course. Soon the mercury stopped running out. The weight of the mercury in the tube was balanced by the pressure of air on the mercury outside.
Mercury barometers are still used today. Scientists read a mercury barometer by seeing how high the column of mercury in the tube is. When air pressure increades, the mercury column goes up. When the pressure decreases the column goes down.
Mercury barometers are very accurate. But they are clumsy to move around, and they have to be kept upright, so that the liquid stays level. That is why ordinary barometers in homes or on ships and airplanes are aneroid barometers. Mercury barometers are used mainly in scientific laboratories.