Richard Dobbs, age 12, of Brownsville, Tex., for his question:
HOW DOES A BAROMETER WORK?
A barometer is an instrument that measures the pressure of the air. It is used chiefly to forecast weather and in measuring heights of mountains.
The barometer's working system is simple. Fill a glass tube that is closed at one end somewhat over 30 inches with mercury. Insert the tube at the open end into a cup of mercury. The level of the mercury will rise and fall depending on the pressure at the surface of the cup.
The first to do this experiment was a pupil of Galileo named Evangelista Torricelli. He did this in 1643 and proved that the weight of all the air above any point on the Earth's surface is equal to the weight of a column of mercury more than 30 inches high.
In 1929, the United States Weather Bureau adopted a new unit of measurement called the bar. Under normal conditions, the bar is taken to be the pressure of 29.53 inches of mercury at sea level. The bureau, by the way, is now called the National Weather Service.
Scientists record the air pressure in millibars, which represent one one thousandth of a bar. This new unit of measurement assures greater accuracy than the old system.
Forecasting the changes in the weather for days ahead was made possible by the barometer. When the mercury falls rapidly in a barometer, a storm is quite likely to follow. A "rising barometer," where the mercury goes up, often foretells better weather.
Using a map, the weather forecaster draws lines connecting the places of equal atmospheric pressure. He can then predict weather changes.
Aneroid barometers are built without liquid. Such barometers have an airtight box containing a partial vacuum. Air pressure causes the box to expand or contract. The barometer needle is attached to the box by a chain, levers and a spring. The needle points out the existing pressure. As the atmospheric pressure above the barometer falls or climbs, the needle points out this change.
Some people with rheumatism or arthritis say that their aches increase when bad weather is approaching. Scientists believe that the decreased pressure of air causes the air in the cells of the body to exert an increased outward pressure. This pressure causes pain in the sensitive tissues of some rheumatic persons. These people become, in effect, barometers.
Aneroid barometers, which are built without liquid, are not as accurate as mercury barometers, but they show much slighter changes in the atmosphere. Aneroid barometers are also easier to carry and are often found in homes and offices where it isn't practical to have 30 inch long tubes of mercury.
A barograph is a barometer that records the atmospheric pressure on a revolving drum. Scientists use it in keeping records of changes in atmospheric pressure over a long period of time.