Kenneth Hodges, age 12, of Eugene, Ore., for his question:
HOW DOES A BAROMETER WORK?
Forecasting the changes in the weather for days ahead was made possible by the barn~
meter. When the mercury falls rapidlY in a barom~/t r, a storm is quite likely to follow. A "rising barometer," where the mercury goes up, often foretells better weather. Using a map, the forecaster draws lines connecting the places of equal atmospheric pressure. He can then predict weather changes.
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.
A barometer's principle is simple. Fill a glass tube somewhat over 30 inches, and closed at one end, with mercury. Put your thumb over the open end and then insert this end into a cup of mercury. The liquid in the tube will fall until its top reaches about 30 inches above the surface of the mercury in the cup.
A pupil of Galileo named Evangelista Torricelli first did this experiment in 1643. From it, he 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 above 30 inches high.
In 1939, 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 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.
Some people with rheumatism 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.
Aneroid barometers are built without liquid. Such a barometer has 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.
Aneroid barometers 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 or offices.
A barograph is a barometer which records the atmospheric pressure on a revolving drum. Scientists use it in keeping records of changes in atmospheric pressure.